Positive electrode active material, power storage device, electronic device, and method for manufacturing positive electrode active material

ABSTRACT

A positive electrode active material includes a plurality of groups of particles. The plurality of groups of particles has a particle diameter of more than or equal to 300 nm and less than or equal to 3 μm. Each of the groups includes two or more particles. The two or more particles are each a lithium-containing complex phosphate including one or more of iron, nickel, manganese, and cobalt. The group of particles includes a first particle and a second particle each having a major diameter and a minor diameter in the upper surface when seen from a predetermined direction. The major diameters of the first and second particles are substantially parallel to each other. The major diameter of the first particle is two to six times larger than the minor diameter of the first particle and the minor diameter of the first particle is more than or equal to 20 nm and less than or equal to 130 nm.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The present invention relates to an object, a method, or a manufacturingmethod. The present invention relates to a process, a machine,manufacture, or a composition of matter. In particular, one embodimentof the present invention relates to a semiconductor device, a displaydevice, a light-emitting device, a power storage device, a memorydevice, a driving method thereof, a manufacturing method thereof, or anevaluation method thereof. In particular, one embodiment of the presentinvention relates to a power storage device, a manufacturing methodthereof, and an evaluation method thereof. Alternatively, the presentinvention relates to a lithium-containing complex phosphate and amanufacturing method thereof. Alternatively, the present inventionrelates to a positive electrode active material and a manufacturingmethod thereof. Alternatively, the present invention relates to alithium ion battery. Alternatively, the present invention relates to abattery management unit and an electronic device.

2. Description of the Related Art

The solubility in a solution at high temperature and under high pressureis higher than at normal temperature and under normal pressure.Furthermore, by controlling pH of the solution, the dissolution andprecipitation of a material can be controlled (Patent Document 1). As anexample of a reaction at high temperature and under high pressure, ahydrothermal method can be given.

In recent years, power storage devices such as lithium-ion secondarybatteries have been developed. Examples of such power storage devicesinclude a power storage device having an electrode formed using lithiumiron phosphate (LiFePO₄), which is a composite oxide, as an activematerial. The power storage device having an electrode formed usingLiFePO₄ has high thermal stability and favorable cycle characteristics.

As an example of a method for generating a composite oxide such asLiFePO₄, the hydrothermal method can be used (e.g., Patent Document 2).

By using the hydrothermal method, even a material which is less likelyto be dissolved in water at normal temperatures and under normalpressures can be dissolved, and thus a substance which is hardlyobtained by a production method performed at normal temperatures andunder normal pressures can be synthesized or crystal growth of such asubstance can be conducted. Furthermore, by using the hydrothermalmethod, microparticles of single crystals of a target substance can beeasily synthesized.

The hydrothermal method, for example, enables a desired compound to begenerated in the following manner: a solution containing a raw materialis introduced into a container resistant to pressure and be subjected topressure treatment and heat treatment; and the treated solution isfiltered.

REFERENCES

[Patent Document 1] PCT International Publication No. 2008/091578

[Patent Document 2] Japanese Published Patent Application No. 2004-95385

SUMMARY OF THE INVENTION

An object of one embodiment of the present invention is to provide acomposite oxide with high diffusion rate of lithium. Another object ofone embodiment of the present invention is to provide a positiveelectrode active material with high diffusion rate of lithium. Anotherobject of one embodiment of the present invention is to provide a powerstorage device with high output. Another object of one embodiment of thepresent invention is to provide a novel power storage device.

Note that the descriptions of these objects do not disturb the existenceof other objects. In one embodiment of the present invention, there isno need to achieve all the objects. Other objects will be apparent fromand can be derived from the description of the specification, thedrawings, the claims, and the like.

A positive electrode active material of one embodiment of the presentinvention includes a plurality of groups of particles. Each of theplurality of groups of particles has a particle diameter of more than orequal to 300 nm and less than or equal to 3 μm. Each of the groupsincludes two or more particles. The two or more particles are each alithium-containing complex phosphate including one or more of iron,nickel, manganese, and cobalt. The first group of particles includes afirst particle and a second particle each having a major diameter and aminor diameter in the upper surface when seen from a predetermineddirection (for example, the upper surface observed with a microscope).The major diameters of the first particle and the second particle aresubstantially parallel to each other when seen from a predetermineddirection. The major diameter of the first particle is two to six timeslarger than the minor diameter of the first particle and the minordiameter of the first particle is more than or equal to 20 nm and lessthan or equal to 130 nm.

A positive electrode active material of one embodiment of the presentinvention includes a plurality of particles. Each of the plurality ofparticles is a lithium-containing complex phosphate including one ormore of iron, nickel, manganese, and cobalt. A first particle and asecond particle of the positive electrode active material each have amajor diameter and a minor diameter in the upper surface observed with amicroscope. The major diameters of the first particle and the secondparticle are substantially parallel to each other when seen from apredetermined direction. The major diameter of the first particle is twoto six times larger than the minor diameter of the first particle andthe minor diameter of the first particle is more than or equal to 20 nmand less than or equal to 130 nm. A median value of the particlediameters obtained with use of a laser diffraction and scattering methodis more than or equal to 500 nm and less than or equal to 6 μm.

A positive electrode active material of one embodiment of the presentinvention includes a plurality of particles. Each of the plurality ofparticles is a lithium-containing complex phosphate including one ormore of iron, nickel, manganese, and cobalt. The first particle and thesecond particle of the positive electrode active material each include amajor diameter and a minor diameter in the upper surface observed with amicroscope. The major diameters of the first particle and the secondparticle are substantially parallel to each other when seen from apredetermined direction. The major diameter of the first particle is twoto six times larger than the minor diameter of the first particle andthe minor diameter of the first particle is more than or equal to 20 nmand less than or equal to 130 nm. A median value of the particlediameters obtained with use of a laser diffraction and scattering methodis more than or equal to 500 nm and less than or equal to 6 μm. Aspecific surface area is more than or equal to 18 m²/g and less than orequal to 50 m²/g.

Furthermore, the above-mentioned positive electrode active materialpreferably has an olivine structure. The above-mentioned positiveelectrode active material is preferably represented by LiFePO₄.

Another embodiment of the present invention is a power storage deviceincluding a positive electrode comprising the positive electrode activematerial described in any one of the above descriptions and a negativeelectrode. Another embodiment of the present invention is an electronicdevice including the power storage device.

A manufacturing method of a positive electrode active material of oneembodiment of the present invention includes a step of mixing a lithiumcompound, a phosphorus compound, and water to form a first mixedsolution, a step of adjusting pH by adding a first aqueous solution tothe first mixed solution to form a second mixed solution, a step ofmixing an iron(II) compound with the second mixed solution to form athird mixed solution, and a step of heating the third mixed solutionunder a pressure higher than or equal to 0.1 MPa and lower than or equalto 2 MPa at a highest temperature higher than 150° C. and lower than orequal to 250° C. to form a fourth mixed solution. The positive electrodeactive material includes a plurality of particles and pH of the thirdmixed solution is more than or equal to 3.5 and less than or equal to5.0. Each of the plurality of particles is a lithium-containing complexphosphate including one or more of iron, nickel, manganese, and cobalt.Each of a first particle and a second particle of the positive electrodeactive material includes a major diameter and a minor diameter in theupper surface observed with a microscope. The major diameters of thefirst particle and the second particle are substantially parallel toeach other when seen from a predetermined direction. The major diametersof the first particle is two to six times larger than the minor diameterof the first particle and the minor diameters of the first particle ismore than or equal to 20 nm and less than or equal to 130 nm. A medianvalue of the particle diameters obtained with use of laser diffractionand scattering method is more than or equal to 500 nm and less than orequal to 6 μm.

A manufacturing method of a positive electrode active material of oneembodiment of the present invention includes a step of mixing a lithiumcompound, a phosphorus compound, and water to form a first mixedsolution, a step of adjusting pH by adding a first aqueous solution tothe first mixed solution to form a second mixed solution, a step ofmixing an iron(II) compound with the second mixed solution to form athird mixed solution, and a step of heating the third mixed solutionunder a pressure higher than or equal to 0.1 MPa and lower than or equalto 2 MPa at a highest temperature higher than 150° C. and lower than orequal to 250° C. to form a fourth mixed solution. The positive electrodeactive material includes a plurality of particles and pH of the thirdmixed solution is more than or equal to 3.5 and less than or equal to5.0. Each of the plurality of particles is a lithium-containing complexphosphate including one or more of iron, nickel, manganese, and cobalt.Each of a first particle and a second particle of the positive electrodeactive material includes a major diameter and a minor diameter in theupper surface observed with a microscope. The major diameter of thefirst particle and the second particle are substantially parallel toeach other when seen from a predetermined direction. The major diametersof the first particle is two to six times larger than the minor diameterof the first particle and the minor diameters of the first particle ismore than or equal to 20 nm and less than or equal to 130 nm. A medianvalue of the particle diameter obtained with use of laser diffractionand scattering method is more than or equal to 500 nm and less than orequal to 6 μm. A specific surface area is more than or equal to 18 m²/gand less than or equal to 50 m²/g.

In the above-mentioned manufacturing method of a positive electrodeactive material, the positive electrode active material preferably hasan olivine structure.

In the above-mentioned manufacturing method of a positive electrodeactive material, the positive electrode active material is preferablyrepresented by LiFePO₄.

One embodiment of the present invention can provide a composite oxidewith high diffusion rate of lithium. Another embodiment of the presentinvention can provide a positive electrode active material with highdiffusion rate of lithium. According to one embodiment of the presentinvention, a power storage device with high output can be provided.Another embodiment of the present invention can provide a novel powerstorage device.

Note that one embodiment of the present invention is not limited tothese effects. For example, depending on circumstances or conditions,one embodiment of the present invention might produce another effect.Furthermore, depending on circumstances or conditions, one embodiment ofthe present invention might not produce any of the above effects.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIGS. 1A and 1B illustrate a positive electrode active material;

FIG. 2 is a flow chart showing a method of manufacturing a positiveelectrode active material;

FIGS. 3A and 3B are diagrams illustrating part of a cross section of anelectrode;

FIG. 4 illustrates a storage battery;

FIGS. 5A and 5B are each a cross-sectional view of a storage battery;

FIGS. 6A and 6B illustrate a method of manufacturing a storage battery;

FIGS. 7A and 7B illustrate a method of manufacturing a storage battery;

FIG. 8 illustrates a storage battery;

FIGS. 9A to 9C are each a diagram for illustrating a radius of curvatureof a surface;

FIGS. 10A to 10D are each a diagram for illustrating a radius ofcurvature of a film;

FIGS. 11A and 11B illustrate a coin-type storage battery;

FIGS. 12A and 12B illustrate a cylindrical storage battery;

FIGS. 13A to 13C are parts of cross-sectional views of a storagebattery;

FIGS. 14A and 14B are parts of cross-sectional views of a storagebattery;

FIGS. 15A to 15C are parts of cross-sectional views of a storagebattery;

FIGS. 16A to 16C illustrate an example of a storage battery;

FIGS. 17A to 17C illustrate an example of a storage battery;

FIGS. 18A and 18B illustrate an example of a power storage system;

FIGS. 19A-1, 19A-2, 19B-1, and 19B-2 illustrate examples of powerstorage systems;

FIGS. 20A and 20B illustrate an example of a power storage system;

FIGS. 21A to 21G illustrate examples of electronic devices;

FIGS. 22A to 22C illustrate an example of an electronic device;

FIG. 23 illustrates examples of electronic device;

FIGS. 24A and 24B illustrate examples of electronic devices;

FIGS. 25A to 25C show a SEM observation result of a positive electrodeactive material;

FIGS. 26A and 26B show a SEM observation result of a positive electrodeactive material;

FIGS. 27A to 27C show a SEM observation result of a positive electrodeactive material;

FIG. 28 shows a major diameter and a minor diameter of a particle; and

FIGS. 29A to 29C show results of particle size distribution measurementof the positive electrode active material.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments of the present invention will be described in detail belowwith reference to the accompanying drawings. However, the presentinvention is not limited to the descriptions of the embodiments and itis easily understood by those skilled in the art that the mode anddetails can be changed variously. Accordingly, the present inventionshould not be interpreted as being limited to the descriptions of theembodiments below.

Note that in drawings used in this specification, the sizes,thicknesses, and the like of components such as films, layers,substrates, and regions are exaggerated for simplicity in some cases.Therefore, the sizes of the components are not limited to the sizes inthe drawings and relative sizes between the components.

Note that the ordinal numbers such as “first” and “second” in thisspecification and the like are used for convenience and do not denotethe order of steps, the stacking order of layers, or the like.Therefore, for example, description can be made even when “first” isreplaced with “second” or “third”, as appropriate. In addition, theordinal numbers in this specification and the like are not necessarilythe same as those which specify one embodiment of the present invention.

Note that in structures of the present invention described in thisspecification and the like, the same portions or portions having similarfunctions are denoted by common reference numerals in differentdrawings, and descriptions thereof are not repeated. Furthermore, thesame hatching pattern is applied to portions having similar functions,and the portions are not especially denoted by reference numerals insome cases.

Note that in this specification and the like, a positive electrode and anegative electrode for a power storage device may be collectivelyreferred to as an electrode; in this case, the electrode refers to atleast one of the positive electrode and the negative electrode.

Embodiment 1

In this embodiment, a positive electrode active material of oneembodiment of the present invention will be described.

A positive electrode active material of one embodiment of the presentinvention includes a plurality of particles. A particle diameter of theparticle included in the positive electrode active material of oneembodiment of the present invention is preferably small. In the positiveelectrode active material of one embodiment of the present invention, itis preferable that the area of particles in contact with each other besmall. The positive electrode active material of one embodiment of thepresent invention preferably has a particle with a high aspect ratio.The positive electrode active material of one embodiment of the presentinvention preferably has a flat particle. The positive electrode activematerial of one embodiment of the present invention is preferablymanufactured using a liquid phase method or more preferably, ahydrothermal method.

[Positive Electrode Active Material]

In the positive electrode active material in a particle form, a traveldistance of carriers is shortened by reducing the particle diameter, sothat the output of the power storage device can be increased. Carriersdiffuse in a one-dimensional direction in the positive electrodematerial having an olivine structure; thus, the output of the powerstorage device can be increased by reducing the thickness in the b-axisdirection which is the moving direction of carriers.

The positive electrode active material having an olivine structure has asmall structure change after lithium is released by discharge, is stablein charge and discharge, offers high safety for the power storagedevice, and has high reliability.

The positive electrode active material of one embodiment of the presentinvention is, for example, a lithium-containing complex phosphateincluding one or more of iron, nickel, manganese, and cobalt.Furthermore, the positive electrode active material of one embodiment ofthe present invention preferably has an olivine structure.

As an example of the lithium-containing complex phosphate having anolivine structure, LiMPO₄ (M is one or more of Fe(II), Ni(II), Co(II),and Mn(II)) can be given. Their specific examples include LiFePO₄,LiNiPO₄, LiCoPO₄, LiMnPO₄, LiFe_(a)Ni_(b)PO₄, LiFe_(a)Co_(b)PO₄,LiFe_(a)Mn_(b)PO₄, LiNi_(a)Co_(b)PO₄, LiNi_(a)Mn_(b)PO₄ (a+b≦1, 0<a<1,and 0<b<1), LiFe_(c)Ni_(d)Co_(e)PO₄, LiFe_(c)Ni_(d)Mn_(e)PO₄,LiNi_(c)Co_(d)Mn_(e)PO₄ (c+d+e≦1, 0<c<1, 0<d<1, and 0<e<1), andLiFe_(f)Ni_(g)Co_(h)Mn_(i)PO₄ (f+g+h+i ≦1, 0<f<1, 0<g<1, 0<h<1, and0<i<1).

The positive electrode active material in a particle form may form agroup of particles. In the group, when the particles are in contact witheach other and an electrolyte cannot enter therebetween, carrier ionscannot reach the surface of the particle; thus the reaction surface areaof the particle is reduced. The reduction of the reaction surface areamay result in a reduction in output of the power storage device.

Therefore, the positive electrode active material preferably has a smallparticle diameter and a space between the particles large enough for theelectrolyte to enter therebetween. For example, in the case where theelectrolytic solution is used, the positive electrode active materialpreferably has a space of more than or equal to 1 nm, preferably morethan or equal to 10 nm to allow the electrolytic solution tosufficiently enter therebetween.

The specific surface area of the particle can be measured by having agas, such as nitrogen absorbed to the surface, for example. Formeasuring the specific surface area, a BET method, a Langmuir method,and the like can be used, for example. The specific surface area isreduced when the particles are in contact with each other and gas cannotenter therebetween. The larger specific surface area is preferable forthe positive electrode active material of one embodiment of the presentinvention.

In the case where a particle has a small specific surface area, thesurface area can be increased in some cases by grinding the particle tobe microparticulated. Note that mechanical grinding of particles cancause damage such as deformation or cracks to the particles in somecases; thus it is not preferable. In the case where the particle ismechanically grinded, the shape of the particle becomes close to aspherical shape, for example. In addition, the aspect ratio decreases.

FIG. 1A illustrates an example of a positive electrode active material201 of one embodiment of the present invention. FIG. 1A shows anobservation result of the surface of the positive electrode activematerial 201 manufactured in Example 1, described later, with use of ascanning electron microscope (SEM). The positive electrode activematerial 201 shown in FIG. 1A includes a particle including lithium ironphosphate.

The positive electrode active material includes a plurality ofparticles. Furthermore, the positive electrode active material of oneembodiment of the present invention preferably includes a plurality ofgroups of particles. The positive electrode active material 201 shown inFIG. 1A includes a group 202 a, a group 202 b, and a group 202 c. FromFIG. 1A, the size of the group is estimated to be approximately 0.6 μm.Here, the group of particles is referred to as a secondary particle insome cases. In any group, 10 or more particles can be observed. Here,the group has a three dimensional shape; thus, in FIG. 1A, the groups202 a, 202 b, and 202 c include particles not only in the observedsurface portion but also in the depth direction which is not observed.Thus, the group includes 30 or more particles, for example.

The particles included in the positive electrode active material of oneembodiment of the present invention forms a group so that the strengthof the positive electrode including the positive electrode activematerial of one embodiment of the present invention can be increased insome cases. On the other hand, in the case where the group of particlesis too large, the uniformity of the thickness of the positive electrodeis reduced in some cases. Furthermore, the conductive additive and theparticles are difficult to be dispersed in some cases.

Thus, the diameter of the group of particles is, for example, preferablyless than or equal to 30 μm, further preferably less than or equal to 10μm, still further preferably more than or equal to 0.1 μm and less thanor equal to 6 μm, and yet further preferably more than or equal to 0.3μm and less than or equal to 3 μm.

Between the particles included in the group, an appropriate space ispreferably provided so that an electrolyte can enter. Thus, a surfacearea of the positive electrode active material is preferably large.

FIG. 1B is a schematic view of the groups. The group 202 a includes aplurality of particles such as a particle 203 a, a particle 203 b, and aparticle 203 c. The group 202 b includes a plurality of particles suchas a particle 204 a and a particle 204 b. The group 202 c includes aplurality of particles such as a particle 205 a, a particle 205 b, and aparticle 205 c. For simplification of the drawing, only parts of theparticles included in the groups of particles are illustrated in FIG.1B.

The major diameter of the particle included in the positive electrodeactive material of one embodiment of the present invention is preferably1.5 to 10 times, 2 to 7 times, or 2 to 6 times the minor diameter.

The ratio of the major diameter to the minor diameter is referred to asan aspect ratio in some cases. A higher aspect ratio leads to easiermanufacturing of the positive electrode in some cases. Furthermore, byincreasing the aspect ratio, the strength of the positive electrode canbe increased in some cases. Furthermore, in the case where the particlehas an olivine structure, a minor diameter direction substantiallyparallel to the b axis leads to a reduction of the diffusion distance oflithium, so that the output of the power storage device can beincreased.

Alternatively, the particle included in the positive electrode activematerial of one embodiment of the present invention has a flat shape. Aflat shape refers to, for example, a thin particle. Alternatively, whenthe particle has a wide surface and a small thickness in a directionsubstantially perpendicular to the surface, a flat shape refers to theparticle with the small thickness. Furthermore, in the case where theparticle has an olivine structure, by having the thickness directionsubstantially parallel to the b axis, the diffusion distance of lithiumcan be reduced so that the output of the power storage device can beincreased.

Here, in the case where the positive electrode active material has anolivine structure, the minor axis preferably goes along the b axisdirection. When the minor axis goes along the b axis, the output of thepower storage device using the positive electrode active material of oneembodiment of the present invention can be increased in some cases.

Here, the major diameter and the minor diameter of the particle may befound by having the particle approximated to an elliptical shape, forexample. Alternatively, for example, the particle is approximated to arectangular solid and the longest side and the shortest side among thethree axes are referred to as the major diameter and the minor diameter,respectively, in some cases.

Alternatively, the minor diameter and the major diameter of a primaryparticle can be estimated by performing surface observation with SEM andellipse approximation. Alternatively, for example, rectangularapproximation is performed in the SEM surface observation and the longside is referred to as the major diameter and the short side is referredto as the minor diameter.

The specific surface area is represented by surface area per weight (theunit is, for example, m²/g). In the case where the shape of the particleis approximated to a sphere shape, the following Formula (1) issatisfied.

$\begin{matrix}\lbrack {{Formula}\mspace{14mu} 1} \rbrack & \; \\{S = {4\pi \; {r^{2} \div ( {\frac{4\pi \; r^{3}}{3} \times d} )}}} & (1)\end{matrix}$

The diameter 2r can be obtained by substituting a specific surface areaS and a density d of the particles into Formula (1). For example, in thecase where the particle is lithium iron phosphate and the density d andthe specific surface area S are 3.55 g/cm³ and 20 m²/g, respectively,the diameter 2r is 124 nm.

Here, in the case where the value of the diameter of the primaryparticle observed with SEM is significantly smaller than the particlediameter calculated from the specific surface area, it is suggested thata large number of primary particles are in contact with each other.

Alternatively, as another method for evaluating the particle diameter,measurement with a particle size distribution analyzer with laserdiffraction and scattering method can be given as an example. Here,information where information of the particle diameter of the particleand that of the diameter of the group of particles are mixed may beobtained in some cases in the evaluation by a particle size distributionanalyzer. For example, information showing the average of the particlediameter of the particle and the diameter of the group of particles canbe obtained in some cases. Note that, for example, in the case where aparticle diameter which is lower than or equal to the lower measurementlimit of the laser diffraction and scattering method exists, informationof a range lower than or equal to the lower measurement limit cannot beobtained.

For example, in the case where the particle diameter obtained bymeasurement with a particle size distribution analyzer using laserdiffraction and scattering method has a value significantly larger thanthe particle diameter obtained from the specific surface area, it issuggested that the particle diameter obtained from the particle sizedistribution analyzer includes information of the diameter of the groupof particles.

The minor diameter of the primary particle is preferably less than orequal to 500 nm, further preferably more than or equal to 10 nm and lessthan or equal to 200 nm, and still further preferably more than or equalto 20 nm and less than or equal to 130 nm in the positive electrodeactive material of one embodiment of the present invention.

The specific surface area of the positive electrode active material ofone embodiment of the present invention is preferably more than or equalto 12 m²/g, further preferably more than or equal to 15 m²/g and lessthan or equal to 40 m²/g, still further preferably more than or equal to18 m²/g and less than or equal to 40 m²/g, and yet further preferablymore than or equal to 20 m²/g and less than or equal to 30 m²/g, forexample.

In the positive electrode active material of one embodiment of thepresent invention, the median value of the particle size (particlediameter) calculated by laser diffraction and scattering method ispreferably less than or equal to 10 μm, more further preferably morethan or equal to 0.5 μm and less than or equal to 6 μm, and stillfurther preferably more than or equal to 0.5 μm and less than or equalto 3 μm.

[Arrangement of Particles]

The particles included in the positive electrode active material of oneembodiment of the present invention are arranged so that the majordiameters of the two or more particles are substantially parallel toeach other when forming the group, for example. Here, FIG. 1B suggeststhat the major diameters of the particles 203 a, 203 b, and 203 cincluded in the group 202 a are substantially parallel to each otheramong the particles, for example. Furthermore, it is suggested that themajor diameters of the particles 204 a and 204 b included in the group202 b are substantially parallel to each other among the particles.Furthermore, it is suggested that the particles 205 a, 205 b, and 205 cincluded in the group 202 c are substantially parallel to each other inthe major diameter direction.

In this manner, the positive electrode active material of one embodimentof the present invention includes a plurality of particles and theparticles are substantially parallel to each other in a major diameterdirection, for example, making more than or equal to 0° and less than orequal to 10° in some cases. The electrode density of the positiveelectrode manufactured using the positive electrode active material ofone embodiment of the present invention can be increased in some caseswhen the plurality of particles are substantially parallel to each otherin a major diameter direction. Thus, the energy density of the powerstorage device can be increased in some cases.

[Manufacturing Method of Positive Electrode Active Material]

The positive electrode active material of one embodiment of the presentinvention is preferably manufactured using a liquid phase method andmore preferably, a hydrothermal method. By using the liquid phasemethod, particles with a small particle diameter can be obtained.Furthermore, by using the liquid phase method, particles with a highaspect ratio can be obtained in some cases. Furthermore, by using thehydrothermal method, productivity can be increased.

The manufacturing method of the positive electrode active material ofone embodiment of the present invention is described with reference toFIG. 2.

In Step S201 a, lithium compound is weighed. In Step S201 b, aphosphorus compound is weighed.

Here, the atomic ratio of lithium to metal M(II) to phosphorus of thelithium-containing complex phosphate preferably obtained as a syntheticmaterial A, described later, is x:y:z. In order to obtain LiMPO₄, forexample, x:y:z=1:1:1 is satisfied.

Typical examples of lithium compound include lithium chloride (LiCl),lithium acetate (CH₃COOLi), lithium oxalate ((COOLi)₂), lithiumcarbonate (Li₂CO₃), and lithium hydroxide monohydrate (LiOH.H₂O).

Typical examples of the phosphorus compound are a phosphoric acid suchas orthophosphoric acid (H₃PO₄), and ammonium hydrogenphosphates such asdiammonium hydrogenphosphate ((NH₄)₂HPO₄) and ammoniumdihydrogenphosphate (NH₄H₂PO₄).

Next, in Step S201 d, a solvent is weighed. Water is preferably used asthe solvent. Furthermore, a mixed solution containing water and anothersolvent may be used as the solvent. For example, water and alcohol maybe mixed. Here, the solubility of lithium compound, phosphorus compound,and a reaction product of lithium compound and phosphorus compound inwater and the solubility thereof in alcohol are different in some cases.By using alcohol, the grain size of the particle, which is to be formed,becomes smaller in some cases. Furthermore, by using alcohol with alower boiling point than water, pressure can be easily increased in somecases in Step S211 described later.

Next, a mixed solution A is formed in Step S205. Mixing can be performedunder an atmosphere of air, inert gas, or the like. As the inert gas,nitrogen may be used, for example. Here, as an example, in an airatmosphere, the solvent weighed in Step S201 d, lithium compound weighedin Step S201 a, and the phosphorus compound weighed in Step S201 b aremixed. For example, lithium compound weighed in Step S201 a and thephosphorus compound weighed in Step S201 b are put in the solventweighed in Step S201 d, so that the mixed solution A is formed. In thecase of forming the mixed solution A in the air atmosphere, an apparatusfor controlling the atmosphere is not necessary, so that the process canbe simplified and cost can be reduced as compared with the case whereinert gas is used.

In the mixed solution A, lithium compound, the phosphorus compound, andthe reaction product of lithium compound and the phosphorus compoundprecipitate, but are partly dissolved without precipitating, i.e.,partly exist in the solvent as ions. Here, when the mixed solution A hasa low pH, there are cases where the reaction product and the like areeasily dissolved in the solvent. When the mixed solution A has a highpH, there are cases where the reaction product and the like are easilyprecipitated in the solvent.

Note that instead of forming the mixed solution A through Step S205, acompound including phosphorus and lithium such as Li₃PO₄, Li₂HPO₄, orLiH₂PO₄ is weighed and added to the solvent so that the mixed solution Amay be formed.

Here, in the case where the mixed solution A is an aqueous solution, pHof the mixed solution A is determined by the type and dissociationdegree of salt included in the mixed solution A. Thus, with lithiumcompound and the phosphorus compound used as source materials, pH of themixed solution A changes. For example, in the case of using lithiumchloride as lithium compound and the orthophosphoric acid as thephosphorus compound, the mixed solution A is a strong acid. Furthermore,for example, in the case where the lithium hydroxide monohydrate is usedas lithium compound, the mixed solution A is likely to be alkaline.

Next, the mixed solution A and a solution Q weighed in Step S205 b aremixed, so that a mixed solution B is formed in Step S207. Here, byadjusting the amount or concentration of the solution Q which is added,pH of the obtained mixed solution B and that of a later obtained mixedsolution C can be adjusted. In Step S207, while pH of the mixed solutionA is measured, the solution Q may be dropped, for example. As thesolution Q, the alkaline solution or the acid solution is used inaccordance with pH of the mixed solution A. By using a slightly alkalinesolution, or a slightly acidic solution, pH is easily adjusted in somecases. For example, a pH of the alkaline solution may be greater than orequal to 8 and less than or equal to 12. Furthermore, a pH of the acidsolution may be greater than or equal to 2 and less than or equal to 6.As the alkaline solution, ammonia water may be used, for example. It ispreferable to determine pH of the solution Q so that the mixed solutionC, which is described later, is acidic or neutral.

In Step S208, one or more of an iron(II) compound, a manganese(II)compound, a cobalt(II) compound, and a nickel(II) compound (hereinafterreferred to as an M(II) compound) are weighed.

Typical examples of the iron(II) compound are iron chloride tetrahydrate(FeCl₂.4H₂O), iron sulfate heptahydrate (FeSO₄.7H₂O), and iron acetate(Fe(CH₃COO)₂).

Typical examples of the manganese(II) compound are manganese chloridetetrahydrate (MnCl₂.4H₂O), manganese sulfate-hydrate (MnSO₄.H₂O), andmanganese acetate tetrahydrate (Mn(CH₃COO)₂.4H₂O).

Typical examples of the cobalt(II) compound are cobalt chloridehexahydrate (CoCl₂.6H₂O), cobalt sulfate heptahydrate (CoSO₄.7H₂O), andcobalt acetate tetrahydrate (Co(CH₃COO)₂.4H₂O).

Typical examples of the nickel(II) compound are nickel chloridehexahydrate (NiCl₂.6H₂O), nickel sulfate hexahydrate (NiSO₄.6H₂O), andnickel acetate tetrahydrate (Ni(CH₃COO)₂.4H₂O).

Next, the mixed solution C is formed in Step S209. Step S209 can beperformed under an atmosphere of air, inert gas, or the like. As theinert gas, nitrogen may be used, for example. Here, as an example, in anair atmosphere, the mixed solution A formed in Step S207 and the M(II)compound weighed in Step S208 are mixed so that the mixed solution C isformed. In the case of performing Step S209 in the air atmosphere, it ispreferable that Step S208 is performed right before Step S209, forexample, within 1 hour, further preferably within 20 minutes, and stillfurther preferably within 10 minutes.

Here, in Step S209, the concentration of the mixed solution C isadjusted by adding a solvent. After a mixture of the mixed solution Band the M(II) compound is formed, the solvent is weighed in Step S209 band the solvent and the mixture are mixed in Step S209 so that the mixedsolution C is manufactured.

Next, in Step S211, the mixed solution C is put into a heat and pressureresistant container such as an autoclave. Heating is performed at atemperature higher than or equal to 100° C. and lower than or equal to350° C., preferably higher than 100° C. and lower than 200° C. and undera pressure higher than or equal to 0.11 MPa and lower than or equal to100 MPa, preferably higher than or equal to 0.11 MPa and lower than orequal to 2 MPa for more than or equal to 0.5 hours and less than orequal to 24 hours, preferably more than or equal to 1 hour and less thanor equal to 10 hours, and further preferably more than or equal to 1hour and less than 5 hours and the solution is then cooled. The solutionin the heat and pressure resistant container is then filtered, followedby washing and drying. After that, the solution is separated. Forexample, filtration and washing are performed. Then, drying is performedin Step S213, and the synthetic material A is obtained.

Here, the lithium-containing complex phosphate, more specifically,LiMPO₄(M is one or more of Fe(II), Ni(II), Co(II), and Mn(II)), forexample, can be preferably obtained as the synthetic material A. As thelithium-containing complex phosphate, LiFePO₄, LiNiPO₄, LiCoPO₄,LiMnPO₄, LiFe_(a)Ni_(b)PO₄, LiFe_(a)Co_(b)PO₄, LiFe_(a)Mn_(b)PO₄,LiNi_(a)Co_(b)PO₄, LiNi_(a)Mn_(b)PO₄ (a+b≦1, 0<a<1, 0<b<1),LiFe_(c)Ni_(d)Co_(e)PO₄, LiFe_(c)Ni_(d)Mn_(e)PO₄,LiNi_(c)Co_(d)Mn_(e)PO₄ (c+d+e≦1, 0<c<1, 0<d<1, 0<e<1),LiFe_(f)Ni_(g)Co_(h)Mn_(i)PO₄ (f+g+h+i≦1, 0<f<1, 0<g<1, 0<h<1, 0<i<1),or the like can be obtained as appropriate depending on the type of theM(II) compound. The lithium-containing complex phosphate obtained inthis embodiment might be a single-crystal grain.

By performing crystal analysis such as XRD or electron diffraction onthe synthetic material A, the crystal structure can be identified. Byperforming crystal analysis on the synthetic material A, a crystalstructure belonging to a space group Pnma can be obtained in some cases.Here, LiMPO₄ having an olivine crystal structure belongs to the spacegroup Pnma, for example.

Here, pH of the mixed solution C is preferably set to more than or equalto 3 and less than or equal to 5 and the highest temperature in StepS211 is set to more than 150° C. and less than 300° C., more preferablymore than or equal to 160° C. and less than 200° C., whereby thesynthetic material A with excellent characteristics with a largespecific surface area and a high aspect ratio can be obtained in somecases. By increasing the reaction temperature, the frequency of thedissolution is increased in contrast with a deposition rate and theadhesion of the particles can be prevented in some cases. For example,by preferably setting pH to more than or equal to 3 and less than orequal to 5 and by preferably setting the highest temperature to morethan 150° C., the adhesion of the particles can be prevented, theparticles form a group, and the particles are arranged so that the majordiameters of the particles are substantially parallel to each other intwo or more particles when the particles form a group.

Embodiment 2

In this embodiment, a storage battery of one embodiment of the presentinvention will be described.

A storage battery of one embodiment of the present invention includes apositive electrode, a negative electrode, and an electrolytic solution.

The positive electrode active material preferably includes the positiveelectrode active material described in Embodiment 1, for example.

[Negative Electrode Active Material]

In the case where the active material is a negative electrode activematerial, for example, an alloy-based material, a carbon-based material,or the like can be used.

For the negative electrode active material, an element which enablescharge-discharge reactions by an alloying reaction and a dealloyingreaction with lithium can be used. For example, a material containing atleast one of silicon, tin, gallium, aluminum, germanium, lead, antimony,bismuth, silver, zinc, cadmium, indium, and the like can be used. Suchelements have higher capacity than carbon. In particular, silicon has ahigh theoretical capacity of 4200 mAh/g. For this reason, silicon ispreferably used as the negative electrode active material.Alternatively, a compound containing any of the above elements may beused. Examples of the compound include SiO, Mg₂Si, Mg₂Ge, SnO, SnO₂,Mg₂Sn, SnS₂, V₂Sn₃, FeSn₂, CoSn₂, Ni₃Sn₂, Cu₆Sn₅, Ag₃Sn, Ag₃Sb, Ni₂MnSb,CeSb₃, LaSn₃, La₃Co₂Sn₇, CoSb₃, InSb, SbSn, and the like. Here, anelement that enables charge-discharge reactions by an alloying reactionand a dealloying reaction with lithium, a compound containing theelement, and the like may be referred to as an alloy-based material.

In this specification and the like, SiO refers, for example, to siliconmonoxide. SiO can alternatively be expressed as SiOx. Here, x preferablyhas an approximate value of 1. For example, x is preferably 0.2 or moreand 1.5 or less, and more preferably 0.3 or more and 1.2 or less.

As the carbon-based material, graphite, graphitizing carbon (softcarbon), non-graphitizing carbon (hard carbon), a carbon nanotube,graphene, carbon black, or the like can be used.

Examples of graphite include artificial graphite and natural graphite.Examples of artificial graphite include meso-carbon microbeads (MCMB),coke-based artificial graphite, and pitch-based artificial graphite. Asartificial graphite, spherical graphite having a spherical shape can beused. For example, MCMB is preferably used because it may have aspherical shape. Moreover, MCMB may preferably be used because it canrelatively easily have a small surface area. Examples of naturalgraphite include flake graphite and spherical natural graphite.

Graphite has a low potential substantially equal to that of a lithiummetal (higher than or equal to 0.05 V and lower than or equal to 0.3 Vvs. Li/Li⁺) when lithium ions are intercalated into the graphite (whilea lithium-graphite intercalation compound is generated). For thisreason, a lithium-ion secondary battery can have a high operatingvoltage. In addition, graphite is preferred because of its advantagessuch as a relatively high capacity per unit volume, relatively smallvolume expansion, low cost, and higher level of safety than that of thelithium metal.

Alternatively, for the negative electrode active materials, an oxidesuch as titanium dioxide (TiO₂), lithium titanium oxide (Li₄Ti₅O₁₂),lithium-graphite intercalation compound (Li_(x)C₆), niobium pentoxide(Nb₂O₅), tungsten oxide (WO₂), or molybdenum oxide (MoO₂) can be used.

Still alternatively, for the negative electrode active materials,Li_(3−x)M_(x)N (M=Co, Ni, or Cu) with a Li₃N structure, which is anitride containing lithium and a transition metal, can be used. Forexample, Li_(2.6)Co_(0.4)N₃ is preferable because of high charge anddischarge capacity (900 mAh/g and 1890 mAh/cm³).

A nitride containing lithium and a transition metal is preferably used,in which case lithium ions are contained in the negative electrodeactive materials and thus the negative electrode active materials can beused in combination with a material for a positive electrode activematerial which does not contain lithium ions, such as V₂O₅ or Cr₃O₈.Note that in the case of using a material including lithium ions as apositive electrode active material, the nitride including lithium and atransition metal can be used for the negative electrode active materialby extracting the lithium ions included in the positive electrode activematerial in advance.

Alternatively, a material which causes a conversion reaction can be usedfor the negative electrode active materials; for example, a transitionmetal oxide which does not form an alloy with lithium, such as cobaltoxide (CoO), nickel oxide (NiO), and iron oxide (FeO), may be used.Other examples of the material which causes a conversion reactioninclude oxides such as Fe₂O₃, CuO, Cu₂O, RuO₂, and Cr₂O₃, sulfides suchas CoS_(0.89), NiS, or CuS, nitrides such as Zn₃N₂, Cu₃N, and Ge₃N₄,phosphides such as NiP₂, FeP₂, and CoP₃, and fluorides such as FeF₃ andBiF₃.

[Predoping]

In the case where a coating film is formed in the initial charge anddischarge cycle, an irreversible reaction occurs. For example, in thecase where one of an irreversible reaction at the positive electrode andan irreversible reaction at the negative electrode is greater than theother, the balance between charge and discharge might be disrupted,resulting in a decrease in the capacity of the storage battery.Replacing an electrode used as a counter electrode after charge anddischarge using the counter electrode are performed can inhibit adecrease in capacity. For example, charge or charge and discharge areperformed using a positive electrode in combination with a negativeelectrode, and then, the positive electrode is removed to be replacedwith another positive electrode in the storage battery. This may inhibita decrease in the capacity of the storage battery. This method may becalled predoping or preaging.

A current collector included in each of the positive electrode and thenegative electrode can be formed using a material that has highconductivity, such as a metal of stainless steel, gold, platinum,aluminum, titanium, or an alloy thereof. In the case where the currentcollector is used in the positive electrode, it is preferred that it notdissolve at the potential of the positive electrode. In the case wherethe current collector is used in the negative electrode, it is preferredthat it not be alloyed with carrier ions such as lithium. Alternatively,an aluminum alloy to which an element which improves heat resistance,such as silicon, titanium, neodymium, scandium, or molybdenum, is addedcan be used. Still alternatively, a metal element which forms silicideby reacting with silicon can be used. Examples of the metal elementwhich forms silicide by reacting with silicon include zirconium,titanium, hafnium, vanadium, niobium, tantalum, chromium, molybdenum,tungsten, cobalt, nickel, and the like. The current collector can haveany of various shapes including a foil-like shape, a plate-like shape(sheet-like shape), a net-like shape, a punching-metal shape, and anexpanded-metal shape. The current collector preferably has a thicknessof more than or equal to 5 μm and less than or equal to 30 μm.

The positive electrode and the negative electrode may include aconductive additive. Examples of the conductive additive include acarbon material, a metal material, and a conductive ceramic material.Alternatively, a fiber material may be used as the conductive additive.The content of the conductive additive in the active material layer ispreferably greater than or equal to 1 wt % and less than or equal to 10wt %, and further preferably greater than or equal to 1 wt % and lessthan or equal to 5 wt %.

A network for electrical conduction can be formed in the electrode bythe conductive additive. The conductive additive also allows maintainingof a path for electric conduction between the positive electrode activematerial particles. The addition of the conductive additive to theactive material layer increases the electric conductivity of the activematerial layer.

Examples of the conductive additive include natural graphite, artificialgraphite such as mesocarbon microbeads, and carbon fiber. Examples ofcarbon fiber include mesophase pitch-based carbon fiber, isotropicpitch-based carbon fiber, carbon nanofiber, and carbon nanotube. Carbonnanotube can be formed by, for example, a vapor deposition method. Otherexamples of the conductive additive include carbon materials such ascarbon black (e.g., acetylene black (AB)), graphite (black lead)particles, graphene, and fullerene. Alternatively, metal powder or metalfibers of copper, nickel, aluminum, silver, gold, or the like, aconductive ceramic material, or the like can be used.

Alternatively, a graphene compound may be used as the conductiveadditive.

A graphene compound may have excellent electrical characteristics ofhigh conductivity and excellent physical properties of high flexibilityand high mechanical strength. A graphene compound has a planar shape andenables low-resistance surface contact. Furthermore, a graphene compoundhas extremely high conductivity even with a small thickness in somecases and thus allows a conductive path to be formed in an activematerial layer efficiently even with a small amount. For this reason, itis preferable to use a graphene compound as the conductive additivebecause the area where the active material and the conductive additiveare in contact with each other can be increased. In addition, it ispreferable to use a graphene compound as the conductive additive becausethe electrical resistance can be reduced in some cases. Here, it isparticularly preferred that graphene, multilayer graphene, or reducedgraphene oxide (hereinafter referred to as RGO), for example, be used asa graphene compound. Note that RGO refers to a compound obtained byreducing graphene oxide (GO), for example.

In the case where an active material with a small particle diameter(e.g., 1 μm or less) is used, the specific surface area of the activematerial is large and thus more conductive paths for the active materialparticles are needed. In such a case, a graphene compound that canefficiently form a conductive path even in a small amount isparticularly preferably used.

A cross-sectional structure example of the active material layer 102containing a graphene compound as a conductive additive will bedescribed below.

FIG. 3A is a longitudinal sectional view of the active material layer102. The active material layer 102 includes active material particles103, graphene compounds 321 as a conductive additive, and a binder (notillustrated). Here, graphene or multilayer graphene can be used as thegraphene compound 321, for example. The graphene compound 321 preferablyhas a sheet-like shape. The graphene compound 321 may have a sheet-likeshape formed of a plurality of sheets of multilayer graphene and/or aplurality of sheets of graphene that partly overlap with each other.

The longitudinal section of the active material layer 102 in FIG. 3Ashows dispersion of the sheet-like graphene compounds 321 in the activematerial layer 102. The graphene compounds 321 are schematically shownby thick lines in FIG. 3A but are actually thin films each having athickness corresponding to the thickness of a single layer or amulti-layer of carbon molecules. The plurality of graphene compounds 321are formed in such a way as to wrap, coat, or adhere to the surfaces ofthe plurality of active material particles 103, so that the graphenecompounds 321 make surface contact with the active material particles103.

Here, a plurality of graphene compounds are bonded to each other to forma net-like graphene compound sheet (hereinafter referred to as agraphene compound net or a graphene net). The graphene net covering theactive material can function as a binder for bonding active materials.The amount of the binder can thus be reduced, or the binder does nothave to be used. This can increase the proportion of the active materialin the electrode volume or weight. That is to say, the capacity of thepower storage device can be increased.

Here, it is preferable to perform reduction after a layer to be theactive material layer 102 is formed in such a manner that graphene oxideis used as the graphene compound 321 and mixed with an active material.When graphene oxide with extremely high dispersibility in a polarsolvent is used for the formation of the graphene compounds 321, thegraphene compounds 321 can be preferably dispersed in the activematerial layer 102. The solvent is removed by volatilization from adispersion medium in which graphene oxide is uniformly dispersed, andthe graphene oxide is reduced; hence, the graphene compounds 321remaining in the active material layer 102 partly overlap with eachother and are dispersed such that surface contact is made, therebyforming a three-dimensional conduction path. Note that graphene oxidecan be reduced either by heat treatment or with the use of a reducingagent, for example.

Unlike a conductive additive in the form of particles, such as acetyleneblack, which makes point contact with an active material, the graphenecompound 321 is capable of making low-resistance surface contact;accordingly, the electrical conduction between the active materialparticles 103 and the graphene compounds 321 can be improved with asmaller amount of the graphene compounds 321 than that of a normalconductive additive. Thus, the proportion of the active materialparticles 103 in the active material layer 102 can be increased.Accordingly, the discharge capacity of a power storage device can beincreased.

The positive electrode and the negative electrode may each include abinder. As the binder, for example, a rubber material such asstyrene-butadiene rubber (SBR), styrene-isoprene-styrene rubber,acrylonitrile-butadiene rubber, butadiene rubber, orethylene-propylene-diene copolymer can be used. Alternatively,fluororubber can be used as the binder.

For the binder, for example, water-soluble polymers are preferably used.As the water-soluble polymers, a polysaccharide or the like can be used.As the polysaccharide, a cellulose derivative such as carboxymethylcellulose (CMC), methyl cellulose, ethyl cellulose, hydroxypropylcellulose, diacetyl cellulose, or regenerated cellulose, starch, or thelike can be used. It is more preferred that such water-soluble polymersbe used in combination with any of the above rubber materials.

Alternatively, as the binder, a material such as polystyrene,poly(methyl acrylate), poly(methyl methacrylate) (PMMA), sodiumpolyacrylate, polyvinyl alcohol (PVA), polyethylene oxide (PEO),polypropylene oxide, polyimide, polyvinyl chloride,polytetrafluoroethylene, polyethylene, polypropylene, polyisobutylene,polyethylene terephthalate, nylon, polyvinylidene fluoride (PVdF),polyacrylonitrile (PAN), ethylene-propylene-diene polymer, polyvinylacetate, or nitrocellulose is preferably used.

Two or more of the above materials may be used in combination for thebinder.

For example, a material having a significant viscosity modifying effectand another material may be used in combination. For example, a rubbermaterial or the like has high adhesion or high elasticity but may havedifficulty in viscosity modification when mixed in a solvent. In such acase, a rubber material or the like is preferably mixed with a materialhaving a significant viscosity modifying effect, for example. As amaterial having a significant viscosity modifying effect, for example, awater-soluble polymer is preferably used. An example of a water-solublepolymer having an especially significant viscosity modifying effect isthe above-mentioned polysaccharide; for example, a cellulose derivativesuch as carboxymethyl cellulose (CMC), methyl cellulose, ethylcellulose, hydroxypropyl cellulose, diacetyl cellulose, or regeneratedcellulose, or starch can be used.

Note that a cellulose derivative such as carboxymethyl cellulose obtainsa higher solubility when converted into a salt such as a sodium salt oran ammonium salt of carboxymethyl cellulose, and accordingly, easilyexerts an effect as a viscosity modifier. The high solubility can alsoincrease the dispersibility of an active material and other componentsin the formation of slurry for an electrode. In this specification,cellulose and a cellulose derivative used as a binder of an electrodeinclude salts thereof.

The water-soluble polymers stabilize viscosity by being dissolved inwater and allow stable dispersion of the active material and anothermaterial combined as a binder such as styrene-butadiene rubber in anaqueous solution. Furthermore, a water-soluble polymer is easily andstably adsorbed to an active material surface because it has afunctional group. It is preferable to use a cellulose derivative becausemany cellulose derivatives such as carboxymethyl cellulose havefunctional groups such as a hydroxyl group and a carboxyl group. Becauseof functional groups, polymers interact with each other and cover anactive material surface in a large area.

The case where the binder covering or being in contact with the activematerial surface forms a film is preferred because the film may as apassivation film to suppress the decomposition of the electrolyticsolution. Here, the passivation film refers to a film without electricconductivity or a film with extremely low electric conductivity, and caninhibit the decomposition of an electrolytic solution at a potential atwhich a battery reaction occurs in the case where the passivation filmis formed on the active material surface, for example. It is preferredthat the passivation film can conduct lithium ions while suppressingelectric conduction.

[Method for Manufacturing Electrode]

In examples of methods for manufacturing negative and positiveelectrodes, a slurry is formed and an electrode is manufactured byapplication of the slurry. A method for forming a slurry used formanufacturing an electrode will be described.

A polar solvent is preferably used as the solvent used for formation ofthe slurry. Examples of the polar solvent include water, methanol,ethanol, acetone, tetrahydrofuran (THF), dimethylformamide (DMF),N-methylpyrrolidone (NMP), dimethyl sulfoxide (DMSO), and a mixedsolution of any two or more of the above.

First, the active material, the conductive additive, and the binder aremixed to form Mixture J. Next, the solvent is added to Mixture J andkneading (mixing with a high viscosity) is performed, so that Mixture Kis formed. Here, Mixture K is preferably in a paste form, for example.In the case where a second binder is added later, a first binder is notnecessarily added in this step in some cases.

Next, the solvent is added to Mixture K and kneading is performed, sothat Mixture L is formed.

Next, in the case where the second binder is used, the second binder isadded to form Mixture M. At this time, a solvent may be added. In thecase where the second binder is not used, a solvent is added as neededto form Mixture N.

Then, Mixture M or Mixture N formed in a reduced-pressure atmosphere iskneaded, for example, to form Mixture O. At this time, a solvent may beadded. In the mixing and kneading steps in each step, a mixer may beused, for example.

Then the viscosity of Mixture O is measured. After that, a solvent isadded as needed to adjust the viscosity. Through the above steps, slurryfor application of the active material layer is obtained.

Here, for example, the higher the viscosity of Mixtures L to O is, thehigher the dispersibility of the active material, the binder, and theconductive additive in the mixtures is (the better they are mixedtogether), in some cases. Thus, the viscosity O is preferably higher.However, an excessively high viscosity of Mixture O is not preferred interms of productivity because it might reduce the electrode applicationspeed.

Next, a method for manufacturing the active material layer over thecurrent collector with the use of the formed slurry will be described.

First, the slurry is applied to the current collector. Before theapplication of the slurry, surface treatment may be performed on thecurrent collector. Examples of surface treatment include coronadischarge treatment, plasma treatment, and undercoat treatment. Here,the “undercoat” refers to a film formed over a current collector beforeapplication of slurry onto the current collector for the purpose ofreducing the interface resistance between an active material layer andthe current collector or increasing the adhesion between the activematerial layer and the current collector. Note that the undercoat is notnecessarily formed in a film shape, and may be formed in an islandshape. In addition, the undercoat may serve as an active material tohave capacity. For the undercoat, a carbon material can be used, forexample. Examples of the carbon material include graphite, carbon blacksuch as acetylene black and ketjen black (registered trademark), and acarbon nanotube.

For the application of the slurry, a slot die method, a gravure method,a blade method, or combination of any of them can be used. Furthermore,a continuous coater or the like may be used for the application.

Then, the solvent of the slurry is volatilized to form the activematerial layer.

The step of volatilizing the solvent of the slurry is preferablyperformed at a temperature in the range from 50° C. to 200° C.inclusive, more preferably from 60° C. to 150° C. inclusive.

Heat treatment is performed using a hot plate at 30° C. or higher and70° C. or lower in an air atmosphere for longer than or equal to 10minutes, and then, for example, another heat treatment is performed atroom temperature or higher and 100° C. or lower in a reduced-pressureenvironment for longer than or equal to 1 hour and shorter than or equalto 10 hours.

Alternatively, heat treatment may be performed using a drying furnace orthe like. In the case of using a drying furnace, the heat treatment isperformed at 30° C. or higher and 120° C. or lower for longer than orequal to 30 seconds and shorter than or equal to 20 minutes, forexample.

The temperature may be increased in stages. For example, after heattreatment is performed at 60° C. or lower for shorter than or equal to10 minutes, another heat treatment may further be performed at higherthan or equal to 65° C. for longer than or equal to 1 minute.

The thickness of the active material layer formed through the abovesteps is, for example, preferably greater than or equal to 5 μm and lessthan or equal to 300 μm, more preferably greater than or equal to 10 μmand less than or equal to 150 μm. Furthermore, the amount of the activematerial in the active material layer 102 is, for example, preferablygreater than or equal to 2 mg/cm² and less than or equal to 50 mg/cm².

The active material layer may be formed over only one surface of thecurrent collector, or the active material layers may be formed such thatthe current collector is sandwiched therebetween. Alternatively, theactive material layers may be formed such that part of the currentcollector is sandwiched therebetween.

After the volatilization of the solvent from the active material layer,pressing may be performed by a compression method such as a roll pressmethod or a flat plate press method. In performing pressing, heat may beapplied.

Note that the active material layer may be predoped. There is noparticular limitation on the method for predoping the active materiallayer. For example, the active material layer may be predopedelectrochemically. For example, before a battery is assembled, theactive material layer can be predoped with lithium in an electrolyticsolution described later with the use of a lithium metal as a counterelectrode. Alternatively, predoping may be performed using a positiveelectrode for predoping as a counter electrode of a negative electrode,and then, the positive electrode for predoping may be removed. Predopingcan particularly inhibit a decrease in initial charge and dischargeefficiency, leading to an increase in the capacity of the storagebattery.

This embodiment can be implemented in combination with any of the otherembodiments as appropriate.

Embodiment 3

In this embodiment, power storage devices of embodiments of the presentinvention will be described.

Examples of the power storage device of one embodiment of the presentinvention include a secondary battery that utilizes an electrochemicalreaction, such as a lithium ion battery, an electrochemical capacitorsuch as an electric double-layer capacitor or a redox capacitor, an airbattery, and a fuel battery.

[Thin Storage Battery]

FIG. 4 illustrates a thin storage battery as an example of a storagedevice. FIG. 4 illustrates an example of a thin storage battery. When aflexible thin storage battery is used in an electronic device at leastpart of which is flexible, the storage battery can be bent as theelectronic device is bent.

FIG. 4 is an external view of a storage battery 500, which is a thinstorage battery. FIG. 5A is a cross-sectional view taken alongdashed-dotted line A1-A2 FIG. 4, and FIG. 5B is a cross-sectional viewtaken along dashed-dotted line B1-B2 in FIG. 4. The storage battery 500includes a positive electrode 503 including a positive electrode currentcollector 501 and a positive electrode active material layer 502, anegative electrode 506 including a negative electrode current collector504 and a negative electrode active material layer 505, a separator 507,an electrolytic solution 508, and an exterior body 509. The separator507 is provided between the positive electrode 503 and the negativeelectrode 506 in the exterior body 509. The electrolytic solution 508 iscontained in the exterior body 509.

As a solvent of the electrolytic solution 508, an aprotic organicsolvent is preferably used. For example, one of ethylene carbonate (EC),propylene carbonate (PC), butylene carbonate, chloroethylene carbonate,vinylene carbonate, γ-butyrolactone, γ-valerolactone, dimethyl carbonate(DMC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC), methylformate, methyl acetate, methyl butyrate, 1,3-dioxane, 1,4-dioxane,dimethoxyethane (DME), dimethyl sulfoxide, diethyl ether, methyldiglyme, acetonitrile, benzonitrile, tetrahydrofuran, sulfolane, andsultone can be used, or two or more of these solvents can be used in anappropriate combination in an appropriate ratio.

When a gelled high-molecular material is used as the solvent of theelectrolytic solution, safety against liquid leakage and the like isimproved. Furthermore, a secondary battery can be thinner and morelightweight. Typical examples of gelled high-molecular materials includea silicone gel, an acrylic gel, an acrylonitrile gel, a polyethyleneoxide-based gel, a polypropylene oxide-based gel, a gel of afluorine-based polymer, and the like.

Alternatively, the use of one or more types of ionic liquids (roomtemperature molten salts) which have features of non-flammability andnon-volatility as a solvent of the electrolytic solution can prevent apower storage device from exploding or catching fire even when a powerstorage device internally shorts out or the internal temperatureincreases owing to overcharging or the like. An ionic liquid contains acation and an anion. The ionic liquid contains an organic cation and ananion. Examples of the organic cation used for the electrolytic solutioninclude aliphatic onium cations such as a quaternary ammonium cation, atertiary sulfonium cation, and a quaternary phosphonium cation, andaromatic cations such as an imidazolium cation and a pyridinium cation.Examples of the anion used for the electrolyte solution include amonovalent amide-based anion, a monovalent methide-based anion, afluorosulfonate anion, a perfluoroalkylsulfonate anion, atetrafluoroborate anion, a perfluoroalkylborate anion, ahexafluorophosphate anion, and a perfluoroalkylphosphate anion.

In the case of using lithium ions as carriers, as an electrolytedissolved in the above-described solvent, one of lithium salts such asLiPF₆, LiClO₄, LiAsF₆, LiBF₄, LiAlCl₄, LiSCN, LiBr, LiI, Li₂SO₄,Li₂B₁₀Cl₁₀ Li₂B₁₂Cl₁₂, LiCF₃SO₃, LiC₄F₉SO₃, LiC(CF₃SO₂)₃, LiC(C₂F₅SO₂)₃,LiN(CF₃SO₂)₂, LiN(C₄F₉SO₂) (CF₃SO₂), and LiN(C₂F₅SO₂)₂ can be used, ortwo or more of these lithium salts can be used in an appropriatecombination in an appropriate ratio.

The electrolytic solution used for a power storage device is preferablyhighly purified and contains a small amount of dust particles andelements other than the constituent elements of the electrolyticsolution (hereinafter, also simply referred to as impurities).Specifically, the weight ratio of impurities to the electrolyticsolution is less than or equal to 1%, preferably less than or equal to0.1%, and more preferably less than or equal to 0.01%.

Furthermore, an additive agent such as vinylene carbonate, propanesultone (PS), tert-butylbenzene (TBB), fluoroethylene carbonate (FEC),or LiBOB may be added to the electrolytic solution. The concentration ofsuch an additive agent in the whole solvent is, for example, higher thanor equal to 0.1 wt % and lower than or equal to 5 wt %.

Alternatively, a polymer gelled electrolyte obtained in such a mannerthat a polymer is swelled with an electrolytic solution may be used.

Examples of polymers include a polymer having a polyalkylene oxidestructure, such as polyethylene oxide (PEO); PVDF; polyacrylonitrile;and a copolymer containing any of them. For example, PVDF-HFP, which isa copolymer of PVDF and hexafluoropropylene (HFP) can be used. Theformed polymer may be porous.

Instead of the electrolytic solution, a solid electrolyte including aninorganic material such as a sulfide-based inorganic material or anoxide-based inorganic material, or a solid electrolyte including ahigh-molecular material such as a polyethylene oxide (PEO)-basedhigh-molecular material may alternatively be used. When the solidelectrolyte is used, a separator and a spacer are not necessary.Furthermore, the battery can be entirely solidified; therefore, there isno possibility of liquid leakage and thus the safety of the battery isdramatically increased.

As the separator 507, paper; nonwoven fabric; glass fiber; ceramics;synthetic fiber containing nylon (polyamide), vinylon (polyvinylalcohol-based fiber), polyester, acrylic, polyolefin, or polyurethane;or the like can be used.

The separator 507 is preferably formed to have a bag-like shape tosurround one of the positive electrode 503 and the negative electrode506. For example, as illustrated in FIG. 6A, the separator 507 is foldedin two so that the positive electrode 503 is sandwiched, and sealed witha sealing portion 514 in a region outside the region overlapping withthe positive electrode 503; thus, the positive electrode 503 can bereliably supported inside the separator 507. Then, as illustrated inFIG. 6B, the positive electrodes 503 surrounded by the separators 507and the negative electrodes 506 are alternately stacked and provided inthe exterior body 509, whereby the storage battery 500 can be formed.

Next, aging after manufacturing a storage battery will be described.Aging is preferably performed after manufacturing of a storage battery.The aging can be performed under the following conditions, for example.Charge is performed at a rate of 0.001 C or more and 0.2 C or less. Thetemperature may be higher than or equal to room temperature and lowerthan or equal to 50° C. In the case where the reaction potential of thepositive electrode or the negative electrode is out of the range of thepotential window of the electrolytic solution 508, the electrolyticsolution is decomposed by charge and discharge operations of a storagebattery in some cases. In the case where the electrolytic solution isdecomposed and a gas is generated and accumulated in the cell, theelectrolytic solution is not in contact with a surface of the electrodein some regions. That is to say, an effectual reaction area of theelectrode is reduced and effectual resistance is increased.

When the resistance is extremely increased, the negative electrodepotential is lowered. Consequently, lithium is intercalated intographite and lithium is deposited on the surface of graphite. Lithiumdeposition might reduce capacity. For example, if a film or the like isgrown on the surface after lithium deposition, lithium deposited on thesurface cannot be dissolved again. This lithium cannot contribute tocapacity. In addition, when deposited lithium is physically collapsedand conduction with the electrode is lost, this lithium also cannotcontribute to capacity. Therefore, the gas is preferably released beforethe negative electrode potential reaches the potential of lithiumbecause of an increase in a charging voltage.

After the release of the gas, the charging state may be maintained at atemperature higher than room temperature, preferably higher than orequal to 30° C. and lower than or equal to 60° C., more preferablyhigher than or equal to 35° C. and lower than or equal to 50° C. for,for example, 1 hour or more and 100 hours or less. In the initialcharge, an electrolytic solution decomposed on the surface forms a filmon a surface of graphite. The formed coating film may thus be densifiedwhen the charging state is held at a temperature higher than roomtemperature after the release of the gas, for example.

FIGS. 7A and 7B illustrate an example where current collectors arewelded to a lead electrode. As illustrated in FIG. 7A, the positiveelectrodes 503 each wrapped by the separator 507 and the negativeelectrodes 506 are alternately stacked. Then, the positive electrodecurrent collectors 501 are welded to a positive electrode lead electrode510, and the negative electrode current collectors 504 are welded to anegative electrode lead electrode 511. FIG. 7B illustrates an example inwhich the positive electrode current collectors 501 are welded to thepositive electrode lead electrode 510. The positive electrode currentcollector 501 is welded to the positive electrode lead electrode 510 ina welding region 512 by ultrasonic welding or the like. The positiveelectrode current collector 501 includes a bent portion 513 asillustrated in FIG. 7B, and it is therefore possible to relieve stressdue to external force applied after manufacturing the storage battery500. The reliability of the storage battery 500 can be thus increased.

In the storage battery 500 illustrated in FIG. 4 and FIGS. 5A and 5B,the positive electrode current collectors 501 in the positive electrode503 and the negative electrode current collectors 504 in the negativeelectrode 506 are welded to the positive electrode lead electrode 510and a negative electrode lead electrode 511, respectively, by ultrasonicwelding. The positive electrode current collector 501 and the negativeelectrode current collector 504 can double as terminals for electricalcontact with the outside. In that case, the positive electrode currentcollector 501 and the negative electrode current collector 504 may bearranged so that part of the positive electrode current collector 501and part of the negative electrode current collector 504 are exposed tothe outside of the exterior body 509 without using lead electrodes.

Although the positive electrode lead electrode 510 and the negativeelectrode lead electrode 511 are provided on the same side in FIG. 4,the positive electrode lead electrode 510 and the negative electrodelead electrode 511 may be provided on different sides as illustrated inFIG. 8. The lead electrodes of a storage battery of one embodiment ofthe present invention can be freely positioned as described above;therefore, the degree of freedom in design is high. Accordingly, aproduct including a storage battery of one embodiment of the presentinvention can have a high degree of freedom in design. Furthermore, ayield of products each including a storage battery of one embodiment ofthe present invention can be increased.

As the exterior body 509 in the storage battery 500, for example, a filmhaving a three-layer structure in which a highly flexible metal thinfilm of aluminum, stainless steel, copper, nickel, or the like isprovided over a film formed of a material such as polyethylene,polypropylene, polycarbonate, ionomer, or polyamide, and an insulatingsynthetic resin film of a polyamide-based resin, a polyester-basedresin, or the like is provided as the outer surface of the exterior bodyover the metal thin film can be used.

Although the examples in FIGS. 5A and 5B each include five positiveelectrode active material layer-negative electrode active material layerpairs (the positive and negative electrode active material layers ofeach pair face each other), it is needless to say that the number ofpairs of active material layers is not limited to five, and may be morethan five or less than five. In the case of using a large number ofactive material layers, the storage battery can have a high capacity. Incontrast, in the case of using a small number of active material layers,the storage battery can have a small thickness and high flexibility.

In the above structure, the exterior body 509 of the secondary batterycan change its form such that the smallest curvature radius is greaterthan or equal to 3 mm and less than or equal to 30 mm, preferablygreater than or equal to 3 mm and less than or equal to 10 mm. One ortwo films are used as the exterior body of the secondary battery. In thecase of a secondary battery having a layered structure, across-sectional structure of the battery that is bent is surrounded bytwo curves of the film serving as the exterior body.

Description will be given of the radius of curvature of a surface withreference to FIGS. 9A to 9C. In FIG. 9A, on a plane 1701 along which acurved surface 1700 is cut, part of a curve 1702 of the curved surface1700 is approximated to an arc of a circle, and the radius of the circleis referred to as a radius 1703 of curvature and the center of thecircle is referred to as a center 1704 of curvature. FIG. 9B is a topview of the curved surface 1700. FIG. 9C is a cross-sectional view ofthe curved surface 1700 taken along the plane 1701. When a curvedsurface is cut by a plane, the radius of curvature of a curve in a crosssection differs depending on the angle between the curved surface andthe plane or on the cut position, and the smallest radius of curvatureis defined as the radius of curvature of a surface in this specificationand the like.

In the case of bending a secondary battery in which a component 1805including electrodes, an electrolytic solution, and the like issandwiched between two films as exterior bodies, a radius 1802 ofcurvature of a film 1801 close to a center 1800 of curvature of thesecondary battery is smaller than a radius 1804 of curvature of a film1803 far from the center 1800 of curvature (FIG. 10A). When thesecondary battery is curved and has an arc-shaped cross section,compressive stress is applied to a surface of the film on the sidecloser to the center 1800 of curvature and tensile stress is applied toa surface of the film on the side far from the center 1800 of curvature(FIG. 10B). However, by forming a pattern including projections ordepressions on surfaces of the exterior bodies, the influence of astrain can be reduced to be acceptable even when compressive stress andtensile stress are applied. For this reason, the secondary battery canchange its form such that the exterior body on the side closer to thecenter of curvature has the smallest curvature radius greater than orequal to 3 mm and less than or equal to 30 mm, preferably greater thanor equal to 3 mm and less than or equal to 10 mm.

Note that the cross-sectional shape of the secondary battery is notlimited to a simple arc shape, and the cross section can be partiallyarc-shaped; for example, a shape illustrated in FIG. 10C, a wavy shapeillustrated in FIG. 10D, and an S shape can be used. When the curvedsurface of the secondary battery has a shape with a plurality of centersof curvature, the secondary battery can change its form such that acurved surface with the smallest radius of curvature among radii ofcurvature with respect to the plurality of centers of curvature, whichis a surface of the exterior body on the side closer to the center ofcurvature, has the smallest curvature radius, for example, greater thanor equal to 3 mm and less than or equal to 30 mm, preferably greaterthan or equal to 3 mm and less than or equal to 10 mm.

Next, a variety of examples of the stack of the positive electrode, thenegative electrode, and the separator will be described.

FIG. 13A illustrates an example where six positive electrodes 111 andsix negative electrodes 115 are stacked. One surface of a positiveelectrode current collector 121 included in a positive electrode 111 isprovided with a positive electrode active material layer 122. Onesurface of a negative electrode current collector 125 included in anegative electrode 115 is provided with a negative electrode activematerial layer 126.

In the structure illustrated in FIG. 13A, the positive electrodes 111and the negative electrodes 115 are stacked so that surfaces of thepositive electrodes 111 on each of which the positive electrode activematerial layer 122 is not provided are in contact with each other andthat surfaces of the negative electrodes 115 on each of which thenegative electrode active material layer 126 is not provided are incontact with each other. When the positive electrodes 111 and thenegative electrodes 115 are stacked in this manner, contact surfacesbetween metals can be formed; specifically, the surfaces of the positiveelectrodes 111 on each of which the positive electrode active materiallayer 122 is not provided can be in contact with each other, and thesurfaces of the negative electrodes 115 on each of which the negativeelectrode active material layer 126 is not provided can be in contactwith each other. The coefficient of friction of the contact surfacebetween metals can be lower than that of a contact surface between theactive material and the separator.

Therefore, when the secondary battery is curved, the surfaces of thepositive electrodes 111 on each of which the positive electrode activematerial layer 122 is not provided slide on each other, and the surfacesof the negative electrodes 115 on each of which the negative electrodeactive material layer 126 is not provided slide on each other; thus, thestress due to the difference between the inner diameter and the outerdiameter of a curved portion can be relieved. Here, the inner diameterof the curved portion refers to the radius of curvature of the innersurface of the curved portion in the exterior body 509 of the storagebattery 500 in the case where the storage battery 500 is curved, forexample. Therefore, the deterioration of the storage battery 500 can beinhibited. Furthermore, the storage battery 500 can have highreliability.

FIG. 13B illustrates an example of a stack of the positive electrodes111 and the negative electrodes 115 which is different from that in FIG.13A. The structure illustrated in FIG. 13B is different from that inFIG. 13A in that the positive electrode active material layers 122 areprovided on both surfaces of the positive electrode current collector121. When the positive electrode active material layers 122 are providedon both the surfaces of the positive electrode current collector 121 asillustrated in FIG. 13B, the capacity per unit volume of the storagebattery 500 can be increased.

FIG. 13C illustrates an example of a stack of the positive electrodes111 and the negative electrodes 115 which is different from that in FIG.13B. The structure illustrated in FIG. 10C is different from that inFIG. 10B in that the negative electrode active material layers 126 areprovided on both surfaces of the negative electrode current collector125. When the negative electrode active material layers 126 are providedon both the surfaces of the negative electrode current collector 125 asillustrated in FIG. 10C, the capacity per unit volume of the storagebattery 500 can be further increased.

In the structures illustrated in FIGS. 13A to 13C, a separator 123 has abag-like shape by which the positive electrodes 111 are surrounded;however, one embodiment of the present invention is not limited thereto.FIG. 14A illustrates an example in which the separator 123 has adifferent structure from that in FIG. 13A. The structure illustrated inFIG. 14A is different from that in FIG. 13A in that a sheet-likeseparator 123 is provided between every pair of the positive electrodeactive material layer 122 and the negative electrode active materiallayer 126. In the structure illustrated in FIG. 14A, six positiveelectrodes 111 and six negative electrodes 115 are stacked, and sixseparators 123 are provided.

FIG. 14B illustrates an example in which the separator 123 differentfrom that in FIG. 14A is provided. The structure illustrated in FIG. 14Bis different from that in FIG. 14A in that one sheet of separator 123 isfolded more than once to be interposed between every pair of thepositive electrode active material layer 122 and the negative electrodeactive material layer 126. It can be said that the structure illustratedin FIG. 14B is a structure in which the separators 123 in the respectivelayers which are illustrated in FIG. 14A are extended and connectedtogether between the layers. In the structure illustrated in FIG. 14B,six positive electrodes 111 and six negative electrodes 115 are stackedand the separator 123 is folded, for example, five times or more. Theseparator 123 is not necessarily provided so as to be interposed betweenevery pair of the positive electrode active material layer 122 and thenegative electrode active material layer 126, and the plurality ofpositive electrodes 111 and the plurality of negative electrodes 115 maybe bound together by extending the separator 123.

Note that the positive electrode, the negative electrode, and theseparator may be stacked as illustrated in FIGS. 15A to 15C. FIG. 15A isa cross-sectional view of a first electrode assembly 130, and FIG. 15Bis a cross-sectional view of a second electrode assembly 131. FIG. 15Cis a cross-sectional view taken along the dashed-dotted line A1-A2 inFIG. 1A. In FIG. 15C, the first electrode assembly 130, the electrodeassembly 131, and the separator 123 are selectively illustrated for thesake of clarity.

As illustrated in FIG. 15C, the storage battery 500 includes a pluralityof first electrode assemblies 130 and a plurality of the electrodeassemblies 131.

As illustrated in FIG. 15A, in each of the first electrode assemblies130, a positive electrode 111 a including the positive electrode activematerial layers 122 on both surfaces of a positive electrode currentcollector 121, the separator 123, a negative electrode 115 a includingthe negative electrode active material layers 126 on both surfaces of anegative electrode current collector 125, the separator 123, and thepositive electrode 111 a including the positive electrode activematerial layers 122 on both surfaces of the positive electrode currentcollector 121 are stacked in this order. As illustrated in FIG. 15B, ineach of the second electrode assemblies 131, the negative electrode 115a including the negative electrode active material layers 126 on bothsurfaces of the negative electrode current collector 125, the separator123, the positive electrode 111 a including the positive electrodeactive material layers 122 on both surfaces of the positive electrodecurrent collector 121, the separator 123, and the negative electrode 115a including the negative electrode active material layers 126 on bothsurfaces of the negative electrode current collector 125 are stacked inthis order.

As illustrated in FIG. 15C, the plurality of first electrode assemblies130 and the plurality of electrode assemblies 131 are covered with thewound separator 123.

[Coin-Type Storage Battery]

Next, an example of a coin-type storage battery will be described as anexample of a power storage device with reference to FIGS. 11A and 11B.FIG. 11A is an external view of a coin-type (single-layer flat type)storage battery, and FIG. 11B is a cross-sectional view thereof.

In a coin-type storage battery 300, a positive electrode can 301doubling as a positive electrode terminal and a negative electrode can302 doubling as a negative electrode terminal are insulated from eachother and sealed by a gasket 303 made of polypropylene or the like. Apositive electrode 304 includes a positive electrode current collector305 and a positive electrode active material layer 306 provided incontact with the positive electrode current collector 305.

A negative electrode 307 includes a negative electrode current collector308 and a negative electrode active material layer 309 provided incontact with the negative electrode current collector 308.

The description of the positive electrode 503 can be referred to for thepositive electrode 304. The description of the positive electrode activematerial layer 502 can be referred to for the positive electrode activematerial layer 306. The description of the negative electrode 506 can bereferred to for the negative electrode 307. The description of thenegative electrode active material layer 505 can be referred to for thenegative electrode active material layer 309. The description of theseparator 507 can be referred to for a separator 310. The description ofthe electrolytic solution 508 can be referred to for the electrolyticsolution.

Note that only one surface of each of the positive electrode 304 and thenegative electrode 307 used for the coin-type storage battery 300 isprovided with an active material layer.

For the positive electrode can 301 and the negative electrode can 302, ametal having a corrosion-resistant property to an electrolytic solution,such as nickel, aluminum, or titanium, an alloy of such a metal, or analloy of such a metal and another metal (e.g., stainless steel or thelike) can be used. Alternatively, the positive electrode can 301 and thenegative electrode can 302 are preferably covered with nickel, aluminum,or the like in order to prevent corrosion due to the electrolyticsolution. The positive electrode can 301 and the negative electrode can302 are electrically connected to the positive electrode 304 and thenegative electrode 307, respectively.

The negative electrode 307, the positive electrode 304, and theseparator 310 are immersed in the electrolytic solution. Then, asillustrated in FIG. 11B, the positive electrode 304, the separator 310,the negative electrode 307, and the negative electrode can 302 arestacked in this order with the positive electrode can 301 positioned atthe bottom, and the positive electrode can 301 and the negativeelectrode can 302 are subjected to pressure bonding with the gasket 303interposed therebetween. In such a manner, the coin-type storage battery300 can be manufactured.

[Cylindrical Storage Battery]

Next, an example of a cylindrical storage battery will be described asan example of a power storage device. The cylindrical storage batterywill be described with reference to FIGS. 12A and 12B. As illustrated inFIG. 12A, a cylindrical storage battery 600 includes a positiveelectrode cap (battery cap) 601 on the upper surface and a battery can(outer can) 602 on the side surface and bottom surface. The positiveelectrode cap and the battery can (outer can) 602 are insulated fromeach other by a gasket (insulating gasket) 610.

FIG. 12B is a diagram schematically illustrating a cross section of thecylindrical storage battery. Inside the battery can 602 having a hollowcylindrical shape, a battery element in which a strip-like positiveelectrode 604 and a strip-like negative electrode 606 are wound with astrip-like separator 605 interposed therebetween is provided. Althoughnot illustrated, the battery element is wound around a center pin. Oneend of the battery can 602 is closed and the other end thereof is open.For the battery can 602, a metal having a corrosion-resistant propertyto an electrolytic solution, such as nickel, aluminum, or titanium, analloy of such a metal, or an alloy of such a metal and another metal(e.g., stainless steel or the like) can be used. Alternatively, thebattery can 602 is preferably covered with nickel, aluminum, or the likein order to prevent corrosion due to the electrolytic solution. Insidethe battery can 602, the battery element in which the positiveelectrode, the negative electrode, and the separator are wound isprovided between a pair of insulating plates 608 and 609 which face eachother. Furthermore, a nonaqueous electrolytic solution (not illustrated)is injected inside the battery can 602 provided with the batteryelement. As the nonaqueous electrolytic solution, a nonaqueouselectrolytic solution that is similar to those of the coin-type storagebattery can be used.

The description of the positive electrode 503 can be referred to for thepositive electrode 604. The description of the negative electrode 506can be referred to for the negative electrode 606. The description ofthe method for manufacturing an electrode that is described inEmbodiment 1 can be referred to for the positive electrode 604 and thenegative electrode 606. Since the positive electrode and the negativeelectrode of the cylindrical storage battery are wound, active materialsare preferably formed on both sides of the current collectors. Apositive electrode terminal (positive electrode current collecting lead)603 is connected to the positive electrode 604, and a negative electrodeterminal (negative electrode current collecting lead) 607 is connectedto the negative electrode 606. Both the positive electrode terminal 603and the negative electrode terminal 607 can be formed using a metalmaterial such as aluminum. The positive electrode terminal 603 and thenegative electrode terminal 607 are resistance-welded to a safety valvemechanism 612 and the bottom of the battery can 602, respectively. Thesafety valve mechanism 612 is electrically connected to the positiveelectrode cap 601 through a positive temperature coefficient (PTC)element 611. The safety valve mechanism 612 cuts off electricalconnection between the positive electrode cap 601 and the positiveelectrode 604 when the internal pressure of the battery exceeds apredetermined threshold value. The PTC element 611, which serves as athermally sensitive resistor whose resistance increases as temperaturerises, limits the amount of current by increasing the resistance, inorder to prevent abnormal heat generation. Note that barium titanate(BaTiO₃)-based semiconductor ceramic or the like can be used for the PTCelement.

In the case where an electrode is wound as in the cylindrical storagebattery illustrated in FIGS. 12A and 12B, a great stress is caused atthe time of winding the electrode. In addition, an outward stress froman axis of winding is applied to the electrode all the time in the casewhere a wound body of the electrode is provided in a housing. However,the active material can be prevented from being cleaved even when such agreat stress is applied to the electrode.

Note that in this embodiment, the coin-type storage battery, thecylindrical storage battery, and the thin storage battery are given asexamples of the storage battery; however, any of storage batteries witha variety of shapes, such as a sealed storage battery and a square-typestorage battery, can be used. Furthermore, a structure in which aplurality of positive electrodes, a plurality of negative electrodes,and a plurality of separators are stacked or a structure in which apositive electrode, a negative electrode, and a separator are wound maybe employed. For example, FIGS. 21A to 21G to FIGS. 29A to 29Cillustrate examples of other storage batteries.

[Structural Example of Thin Storage Battery]

FIGS. 16A to 16C and FIGS. 17A to 17C illustrate structural examples ofthin storage batteries. A wound body 993 illustrated in FIG. 16Aincludes a negative electrode 994, a positive electrode 995, and aseparator 996.

The wound body 993 is obtained by winding a sheet of a stack in whichthe negative electrode 994 overlaps with the positive electrode 995 withthe separator 996 provided therebetween. The wound body 993 is coveredwith a rectangular sealed container or the like; thus, a rectangularsecondary battery is manufactured.

Note that the number of stacks each including the negative electrode994, the positive electrode 995, and the separator 996 is determined asappropriate depending on capacity and element volume which are required.The negative electrode 994 is connected to a negative electrode currentcollector (not illustrated) via one of a lead electrode 997 and a leadelectrode 998. The positive electrode 995 is connected to a positiveelectrode current collector (not illustrated) via the other of the leadelectrode 997 and the lead electrode 998.

In a storage battery 990 illustrated in FIGS. 16B and 16C, the woundbody 993 is packed in a space formed by bonding a film 981 and a film982 having a depressed portion that serve as exterior bodies bythermocompression bonding or the like. The wound body 993 includes thelead electrode 997 and the lead electrode 998, and is soaked in anelectrolytic solution inside a space surrounded by the film 981 and thefilm 982 having a depressed portion.

For the film 981 and the film 982 having a depressed portion, a metalmaterial such as aluminum or a resin material can be used, for example.With the use of a resin material for the film 981 and the film 982having a depressed portion, the film 981 and the film 982 having adepressed portion can be changed in their forms when external force isapplied; thus, a flexible storage battery can be manufactured.

Although FIGS. 16B and 16C illustrate an example where a space is formedby two films, the wound body 993 may be placed in a space formed bybending one film.

Furthermore, in manufacturing a flexible power storage device, a resinmaterial or the like can be used for an exterior body and a sealedcontainer of the power storage device. Note that in the case where aresin material is used for the exterior body and the sealed container, aconductive material is used for a portion connected to the outside.

For example, FIGS. 17A to 17C illustrate another example of a flexiblethin storage battery. The wound body 993 illustrated in FIG. 17A is thesame as that illustrated in FIG. 16A, and the detailed descriptionthereof is omitted.

In the storage battery 990 illustrated in FIGS. 17B and 17C, the woundbody 993 is packed in an exterior body 991. The wound body 993 includesthe lead electrode 997 and the lead electrode 998, and is soaked in anelectrolytic solution inside a space surrounded by the exterior body 991and an exterior body 992. For example, a metal material such as aluminumor a resin material can be used for the exterior bodies 991 and 992.With the use of a resin material for the exterior bodies 991 and 992,the exterior bodies 991 and 992 can be changed in their forms whenexternal force is applied; thus, a flexible thin storage battery can bemanufactured.

When the electrode including the active material of one embodiment ofthe present invention is used in the flexible thin storage battery, theactive material can be prevented from being cleaved even if a stresscaused by repeated bending of the thin storage battery is applied to theelectrode.

When the active material in which at least part of the cleavage plane iscovered with graphene is used for an electrode as described above, adecrease in the voltage and discharge capacity of a battery can beprevented. Accordingly, the charge-discharge cycle characteristics ofthe battery can be improved.

[Structural Example of Power Storage System]

Structural examples of power storage systems will be described withreference to FIGS. 18A and 18B to FIGS. 20A and 20B. Here, a powerstorage system refers to, for example, a device including a powerstorage device.

FIGS. 18A and 18B are external views of a power storage system. Thepower storage system includes a circuit board 900 and a storage battery913. A label 910 is attached to the storage battery 913. As shown inFIG. 18B, the power storage system further includes a terminal 951, aterminal 952, an antenna 914, and an antenna 915.

The circuit board 900 includes terminals 911 and a circuit 912. Theterminals 911 are connected to the terminals 951 and 952, the antennas914 and 915, and the circuit 912. Note that a plurality of terminals 911serving as a control signal input terminal, a power supply terminal, andthe like may be provided.

The circuit 912 may be provided on the rear surface of the circuit board900. The shape of each of the antennas 914 and 915 is not limited to acoil shape and may be a linear shape or a plate shape. Furthermore, aplanar antenna, an aperture antenna, a traveling-wave antenna, an EHantenna, a magnetic-field antenna, or a dielectric antenna may be used.Alternatively, the antenna 914 or the antenna 915 may be a flat-plateconductor. The flat-plate conductor can serve as one of conductors forelectric field coupling. That is, the antenna 914 or the antenna 915 canserve as one of two conductors of a capacitor. Thus, electric power canbe transmitted and received not only by an electromagnetic field or amagnetic field but also by an electric field.

The line width of the antenna 914 is preferably larger than that of theantenna 915. This makes it possible to increase the amount of electricpower received by the antenna 914.

The power storage system includes a layer 916 between the storagebattery 913 and the antennas 914 and 915. The layer 916 may have afunction of blocking an electromagnetic field by the storage battery913. As the layer 916, for example, a magnetic body can be used.

Note that the structure of the power storage system is not limited tothat shown in FIGS. 18A and 18B.

For example, as shown in FIGS. 19A-1 and 19A-2, two opposite surfaces ofthe storage battery 913 in FIGS. 18A and 18B may be provided withrespective antennas. FIG. 19A-1 is an external view showing one side ofthe opposite surfaces, and FIG. 19A-2 is an external view showing theother side of the opposite surfaces. For portions similar to those inFIGS. 18A and 18B, the description of the power storage systemillustrated in FIGS. 18A and 18B can be referred to as appropriate.

As illustrated in FIG. 19A-1, the antenna 914 is provided on one of theopposite surfaces of the storage battery 913 with the layer 916interposed therebetween, and as illustrated in FIG. 19A-2, the antenna915 is provided on the other of the opposite surfaces of the storagebattery 913 with a layer 917 interposed therebetween. The layer 917 mayhave a function of blocking an electromagnetic field by the storagebattery 913. As the layer 917, for example, a magnetic body can be used.

With the above structure, both of the antennas 914 and 915 can beincreased in size.

Alternatively, as illustrated in FIGS. 19B-1 and 19B-2, two oppositesurfaces of the storage battery 913 in FIGS. 18A and 18B may be providedwith different types of antennas. FIG. 19B-1 is an external view showingone side of the opposite surfaces, and FIG. 19B-2 is an external viewshowing the other side of the opposite surfaces. For portions similar tothose in FIGS. 18A and 18B, the description of the power storage systemillustrated in FIGS. 18A and 18B can be referred to as appropriate.

As illustrated in FIG. 19B-1, the antennas 914 and 915 are provided onone of the opposite surfaces of the storage battery 913 with the layer916 interposed therebetween, and as illustrated in FIG. 19B-2, anantenna 918 is provided on the other of the opposite surfaces of thestorage battery 913 with the layer 917 interposed therebetween. Theantenna 918 has a function of communicating data with an externaldevice, for example. An antenna with a shape that can be applied to theantennas 914 and 915, for example, can be used as the antenna 918. As asystem for communication using the antenna 918 between the power storagesystem and another device, a response method that can be used betweenthe power storage system and another device, such as NFC, can beemployed.

Alternatively, as illustrated in FIG. 20A, the storage battery 913 inFIGS. 18A and 18B may be provided with a display device 920. The displaydevice 920 is electrically connected to the terminal 911 via a terminal919. It is possible that the label 910 is not provided in a portionwhere the display device 920 is provided. For portions similar to thosein FIGS. 18A and 18B, the description of the power storage systemillustrated in FIGS. 18A and 18B can be referred to as appropriate.

The display device 920 can display, for example, an image showingwhether charge is being carried out, an image showing the amount ofstored power, or the like. As the display device 920, electronic paper,a liquid crystal display device, an electroluminescent (EL) displaydevice, or the like can be used. For example, the use of electronicpaper can reduce power consumption of the display device 920.

Alternatively, as illustrated in FIG. 20B, the storage battery 913illustrated in FIGS. 18A and 18B may be provided with a sensor 921. Thesensor 921 is electrically connected to the terminal 911 via a terminal922. For portions similar to those in FIGS. 18A and 18B, the descriptionof the power storage system illustrated in FIGS. 18A and 18B can bereferred to as appropriate.

As the sensor 921, a sensor that has a function of measuring, forexample, force, displacement, position, speed, acceleration, angularvelocity, rotational frequency, distance, light, liquid, magnetism,temperature, chemical substance, sound, time, hardness, electric field,electric current, voltage, electric power, radiation, flow rate,humidity, gradient, oscillation, odor, or infrared rays can be used.With the sensor 921, for example, data on an environment (e.g.,temperature) where the power storage system is placed can be determinedand stored in a memory inside the circuit 912.

The electrode of one embodiment of the present invention is used in thestorage battery and the power storage system of one embodiment of thepresent invention. Thus, the capacity of the storage battery and thepower storage system can be high. Furthermore, the energy density can behigh. Moreover, reliability can be high, and life can be long.

This embodiment can be implemented in appropriate combination with anyof the other embodiments.

Embodiment 4

In this embodiment, an example of an electronic device including aflexible storage battery will be described.

FIGS. 21A to 21G illustrate examples of electronic devices including theflexible power storage device described in Embodiment 2. Examples ofelectronic devices each including a flexible power storage deviceinclude television devices (also referred to as televisions ortelevision receivers), monitors of computers or the like, cameras suchas digital cameras and digital video cameras, digital photo frames,mobile phones (also referred to as mobile phones or mobile phonedevices), portable game machines, portable information terminals, audioreproducing devices, and large game machines such as pachinko machines.

In addition, a flexible power storage device can be incorporated along acurved inside/outside wall surface of a house or a building or a curvedinterior/exterior surface of a car.

FIG. 21A is an example of a mobile phone. A mobile phone 7400 isprovided with a display portion 7402 incorporated in a housing 7401, anoperation button 7403, an external connection port 7404, a speaker 7405,a microphone 7406, and the like. Note that the mobile phone 7400includes a power storage device 7407.

FIG. 21B illustrates the mobile phone 7400 that is curved. When thewhole mobile phone 7400 is bent by the external force, the power storagedevice 7407 included in the mobile phone 7400 is also bent. FIG. 21Cillustrates the bent power storage device 7407. The power storage device7407 is a thin storage battery. The power storage device 7407 is fixedin a state of being bent. Note that the power storage device 7407includes a lead electrode 7408 electrically connected to a currentcollector 7409. The current collector 7409 is, for example, copper foil,and partly alloyed with gallium; thus, adhesion between the currentcollector 7409 and an active material layer in contact with the currentcollector 7409 is improved and the power storage device 7407 can havehigh reliability even in a state of being bent.

FIG. 21D illustrates an example of a bangle display device. A portabledisplay device 7100 includes a housing 7101, a display portion 7102, anoperation button 7103, and a power storage device 7104. FIG. 21Eillustrates the bent power storage device 7104. When the display deviceis worn on a user's arm while the power storage device 7104 is bent, thehousing changes its form and the curvature of a part or the whole of thepower storage device 7104 is changed. Note that the radius of curvatureof a curve at a point refers to the radius of the circular arc that bestapproximates the curve at that point. The reciprocal of the radius ofcurvature is curvature. Specifically, a part or the whole of the housingor the main surface of the power storage device 7104 is changed in therange of radius of curvature from 40 mm to 150 mm inclusive. When theradius of curvature at the main surface of the power storage device 7104is greater than or equal to 40 mm and less than or equal to 150 mm, thereliability can be kept high.

FIG. 21F illustrates an example of a watch-type portable informationterminal. A portable information terminal 7200 includes a housing 7201,a display portion 7202, a band 7203, a buckle 7204, an operation button7205, an input output terminal 7206, and the like.

The portable information terminal 7200 is capable of executing a varietyof applications such as mobile phone calls, e-mailing, viewing andediting texts, music reproduction, Internet communication, and acomputer game.

The display surface of the display portion 7202 is curved, and imagescan be displayed on the curved display surface. In addition, the displayportion 7202 includes a touch sensor, and operation can be performed bytouching the screen with a finger, a stylus, or the like. For example,by touching an icon 7207 displayed on the display portion 7202,application can be started.

With the operation button 7205, a variety of functions such as timesetting, power on/off, on/off of wireless communication, setting andcancellation of a silent mode, and setting and cancellation of a powersaving mode can be performed. For example, the functions of theoperation button 7205 can be set freely by setting the operation systemincorporated in the portable information terminal 7200.

The portable information terminal 7200 can employ near fieldcommunication that is a communication method based on an existingcommunication standard. In that case, for example, mutual communicationbetween the portable information terminal 7200 and a headset capable ofwireless communication can be performed, and thus hands-free calling ispossible.

Moreover, the portable information terminal 7200 includes the inputoutput terminal 7206, and data can be directly transmitted to andreceived from another information terminal via a connector. In addition,charging via the input output terminal 7206 is possible. Note that thecharging operation may be performed by wireless power feeding withoutusing the input output terminal 7206.

The display portion 7202 of the portable information terminal 7200 isprovided with a power storage device including the electrode of oneembodiment of the present invention. For example, the power storagedevice 7104 illustrated in FIG. 21E that is in the state of being curvedcan be provided in the housing 7201. Alternatively, the power storagedevice 7104 illustrated in FIG. 21E can be provided in the band 7203such that it can be curved.

The portable information terminal 7200 preferably includes a sensor. Asthe sensor, for example a human body sensor such as a fingerprintsensor, a pulse sensor, or a temperature sensor, a touch sensor, apressure sensitive sensor, an acceleration sensor, or the like ispreferably mounted.

FIG. 21G illustrates an example of an armband display device. A displaydevice 7300 includes a display portion 7304 and the power storage deviceof one embodiment of the present invention. The display device 7300 caninclude a touch sensor in the display portion 7304 and can serve as aportable information terminal.

The display surface of the display portion 7304 is bent, and images canbe displayed on the bent display surface. A display state of the displaydevice 7300 can be changed by, for example, near field communication,which is a communication method based on an existing communicationstandard.

The display device 7300 includes an input output terminal, and data canbe directly transmitted to and received from another informationterminal via a connector. In addition, charging via the input outputterminal is possible. Note that the charging operation may be performedby wireless power feeding without using the input output terminal.

This embodiment can be combined with any of the other embodiments asappropriate.

Embodiment 5

In this embodiment, examples of electronic devices that can includepower storage devices will be described.

FIGS. 22A and 22B illustrate an example of a tablet terminal that can befolded in half. A tablet terminal 9600 illustrated in FIGS. 22A and 22Bincludes a housing 9630 a, a housing 9630 b, a movable portion 9640connecting the housings 9630 a and 9630 b, a display portion 9631including a display portion 9631 a and a display portion 9631 b, adisplay mode changing switch 9626, a power switch 9627, a power savingmode changing switch 9625, a fastener 9629, and an operation switch9628. FIG. 22A illustrates the tablet terminal 9600 that is opened, andFIG. 22B illustrates the tablet terminal 9600 that is closed.

The tablet terminal 9600 includes a power storage unit 9635 inside thehousings 9630 a and 9630 b. The power storage unit 9635 is providedacross the housings 9630 a and 9630 b, passing through the movableportion 9640.

Part of the display portion 9631 a can be a touch panel region 9632 aand data can be input when a displayed operation key 9638 is touched.Although a structure in which a half region in the display portion 9631a has only a display function and the other half region has a touchpanel function is shown as an example, the display portion 9631 a is notlimited to the structure. The whole region in the display portion 9631 amay have a touch panel function. For example, the display portion 9631 acan display keyboard buttons in the whole region to be a touch panel,and the display portion 9631 b can be used as a display screen.

In the display portion 9631 b, as in the display portion 9631 a, part ofthe display portion 9631 b can be a touch panel region 9632 b. Aswitching button 9639 for showing/hiding a keyboard of the touch panelis touched with a finger, a stylus, or the like, so that keyboardbuttons can be displayed on the display portion 9631 b.

Touch input can be performed in the touch panel region 9632 a and thetouch panel region 9632 b at the same time.

The display mode switch 9626 can switch the display between a portraitmode and a landscape mode, and between monochrome display and colordisplay, for example. The power saving mode changing switch 9625 cancontrol display luminance in accordance with the amount of externallight in use of the tablet terminal 9600, which is measured with anoptical sensor incorporated in the tablet terminal 9600. Anotherdetection device including a sensor for detecting inclination, such as agyroscope sensor or an acceleration sensor, may be incorporated in thetablet terminal, in addition to the optical sensor.

Although the display area of the display portion 9631 a is the same asthat of the display portion 9631 b in FIG. 22A, one embodiment of thepresent invention is not particularly limited thereto. The display areaof the display portion 9631 a may be different from that of the displayportion 9631 b, and furthermore, the display quality of the displayportion 9631 a may be different from that of the display portion 9631 b.For example, one display panel may be capable of higher-definitiondisplay than the other display panel.

The tablet terminal is closed in FIG. 22B. The tablet terminal includesthe housing 9630, a solar cell 9633, and a charge and discharge controlcircuit 9634 including a DCDC converter 9636. The power storage unit ofone embodiment of the present invention is used as the power storageunit 9635.

The tablet terminal 9600 can be folded in two such that the housings9630 a and 9630 b overlap with each other when not in use. Thus, thedisplay portions 9631 a and 9631 b can be protected, which increases thedurability of the tablet terminal 9600. In addition, the power storageunit 9635 of one embodiment of the present invention has flexibility andcan be repeatedly bent without a significant decrease in charge anddischarge capacity. Thus, a highly reliable tablet terminal can beprovided.

The tablet terminal illustrated in FIGS. 22A and 22B can also have afunction of displaying various kinds of data (e.g., a still image, amoving image, and a text image), a function of displaying a calendar, adate, or the time on the display portion, a touch-input function ofoperating or editing data displayed on the display portion by touchinput, a function of controlling processing by various kinds of software(programs), and the like.

The solar battery 9633, which is attached on the surface of the tabletterminal, supplies electric power to a touch panel, a display portion,an image signal processor, and the like. Note that the solar cell 9633can be provided on one or both surfaces of the housing 9630 and thepower storage unit 9635 can be charged efficiently. The use of alithium-ion battery as the power storage unit 9635 brings an advantagesuch as reduction in size.

The structure and operation of the charge and discharge control circuit9634 illustrated in FIG. 22B will be described with reference to a blockdiagram in FIG. 22C. The solar cell 9633, the power storage unit 9635,the DCDC converter 9636, a converter 9637, switches SW1 to SW3, and thedisplay portion 9631 are illustrated in FIG. 22C, and the power storageunit 9635, the DCDC converter 9636, the converter 9637, and the switchesSW1 to SW3 correspond to the charge and discharge control circuit 9634in FIG. 22B.

First, an example of the operation in the case where power is generatedby the solar cell 9633 using external light is described. The voltage ofelectric power generated by the solar cell is raised or lowered by theDCDC converter 9636 to a voltage for charging the power storage unit9635. When the power from the solar battery 9633 is used for theoperation of the display portion 9631, the switch SW1 is turned on andthe voltage of the power is raised or lowered by the converter 9637 to avoltage needed for operating the display portion 9631. When display onthe display portion 9631 is not performed, the switch SW1 is turned offand the switch SW2 is turned on, so that the power storage unit 9635 canbe charged.

Note that the solar cell 9633 is described as an example of a powergeneration means; however, one embodiment of the present invention isnot limited to this example. The power storage unit 9635 may be chargedusing another power generation means such as a piezoelectric element ora thermoelectric conversion element (Peltier element). For example, thepower storage unit 9635 may be charged with a non-contact powertransmission module capable of performing charging by transmitting andreceiving electric power wirelessly (without contact), or any of theother charge means used in combination.

FIG. 23 illustrates other examples of electronic devices. In FIG. 23, adisplay device 8000 is an example of an electronic device including apower storage device 8004 of one embodiment of the present invention.Specifically, the display device 8000 corresponds to a display devicefor TV broadcast reception and includes a housing 8001, a displayportion 8002, speaker portions 8003, and the power storage device 8004.The power storage device 8004 of one embodiment of the present inventionis provided in the housing 8001. The display device 8000 can receiveelectric power from a commercial power supply. Alternatively, thedisplay device 8000 can use electric power stored in the power storagedevice 8004. Thus, the display device 8000 can be operated with the useof the power storage device 8004 of one embodiment of the presentinvention as an uninterruptible power supply even when electric powercannot be supplied from a commercial power supply due to power failureor the like.

A semiconductor display device such as a liquid crystal display device,a light-emitting device in which a light-emitting element such as anorganic EL element is provided in each pixel, an electrophoretic displaydevice, a digital micromirror device (DMD), a plasma display panel(PDP), or a field emission display (FED) can be used for the displayportion 8002.

Note that the display device includes, in its category, all ofinformation display devices for personal computers, advertisementdisplays, and the like other than TV broadcast reception.

In FIG. 23, an installation lighting device 8100 is an example of anelectronic device including a power storage device 8103 of oneembodiment of the present invention. Specifically, the lighting device8100 includes a housing 8101, a light source 8102, and the power storagedevice 8103. Although FIG. 23 illustrates the case where the powerstorage device 8103 is provided in a ceiling 8104 on which the housing8101 and the light source 8102 are installed, the power storage device8103 may be provided in the housing 8101. The lighting device 8100 canreceive electric power from a commercial power supply. Alternatively,the lighting device 8100 can use electric power stored in the powerstorage device 8103. Thus, the lighting device 8100 can be operated withthe use of power storage device 8103 of one embodiment of the presentinvention as an uninterruptible power supply even when electric powercannot be supplied from a commercial power supply due to power failureor the like.

Note that although the installation lighting device 8100 provided in theceiling 8104 is illustrated in FIG. 23 as an example, the power storagedevice of one embodiment of the present invention can be used in aninstallation lighting device provided in, for example, a wall 8105, afloor 8106, a window 8107, or the like other than the ceiling 8104.Alternatively, the power storage device of one embodiment of the presentinvention can be used in a tabletop lighting device or the like.

As the light source 8102, an artificial light source which emits lightartificially by using electric power can be used. Specifically, anincandescent lamp, a discharge lamp such as a fluorescent lamp, andlight-emitting elements such as an LED and an organic EL element aregiven as examples of the artificial light source.

In FIG. 23, an air conditioner including an indoor unit 8200 and anoutdoor unit 8204 is an example of an electronic device including apower storage device 8203 of one embodiment of the present invention.Specifically, the indoor unit 8200 includes a housing 8201, an airoutlet 8202, and the power storage device 8203. Although FIG. 23illustrates the case where the power storage device 8203 is provided inthe indoor unit 8200, the power storage device 8203 may be provided inthe outdoor unit 8204. Alternatively, the power storage devices 8203 maybe provided in both the indoor unit 8200 and the outdoor unit 8204. Theair conditioner can receive electric power from a commercial powersupply. Alternatively, the air conditioner can use electric power storedin the power storage device 8203. Particularly in the case where thepower storage devices 8203 are provided in both the indoor unit 8200 andthe outdoor unit 8204, the air conditioner can be operated with the useof the power storage device 8203 of one embodiment of the presentinvention as an uninterruptible power supply even when electric powercannot be supplied from a commercial power supply due to power failureor the like.

Note that although the split-type air conditioner including the indoorunit and the outdoor unit is illustrated in FIG. 23 as an example, thepower storage device of one embodiment of the present invention can beused in an air conditioner in which the functions of an indoor unit andan outdoor unit are integrated in one housing.

In FIG. 23, an electric refrigerator-freezer 8300 is an example of anelectronic device including a power storage device 8304 of oneembodiment of the present invention. Specifically, the electricrefrigerator-freezer 8300 includes a housing 8301, a door for arefrigerator 8302, a door for a freezer 8303, and the power storagedevice 8304. The power storage device 8304 is provided in the housing8301 in FIG. 23. The electric refrigerator-freezer 8300 can receiveelectric power from a commercial power supply. Alternatively, theelectric refrigerator-freezer 8300 can use electric power stored in thepower storage device 8304. Thus, the electric refrigerator-freezer 8300can be operated with the use of the power storage device 8304 of oneembodiment of the present invention as an uninterruptible power supplyeven when electric power cannot be supplied from a commercial powersupply due to power failure or the like.

Note that among the electronic devices described above, a high-frequencyheating apparatus such as a microwave oven and an electronic device suchas an electric rice cooker require high power in a short time. Thetripping of a breaker of a commercial power supply in use of anelectronic device can be prevented by using the power storage device ofone embodiment of the present invention as an auxiliary power supply forsupplying electric power which cannot be supplied enough by a commercialpower supply.

In addition, in a time period when electronic devices are not used,particularly when the proportion of the amount of electric power whichis actually used to the total amount of electric power which can besupplied from a commercial power supply source (such a proportionreferred to as a usage rate of electric power) is low, electric powercan be stored in the power storage device, whereby the usage rate ofelectric power can be reduced in a time period when the electronicdevices are used. For example, in the case of the electricrefrigerator-freezer 8300, electric power can be stored in the powerstorage device 8304 in night time when the temperature is low and thedoor for a refrigerator 8302 and the door for a freezer 8303 are notoften opened or closed. On the other hand, in daytime when thetemperature is high and the door for a refrigerator 8302 and the doorfor a freezer 8303 are frequently opened and closed, the power storagedevice 8304 is used as an auxiliary power supply; thus, the usage rateof electric power in daytime can be reduced.

This embodiment can be combined with any of the other embodiments asappropriate.

Embodiment 6

In this embodiment, examples of vehicles using power storage deviceswill be described.

The use of power storage devices in vehicles enables production ofnext-generation clean energy vehicles such as hybrid electric vehicles(HEVs), electric vehicles (EVs), and plug-in hybrid electric vehicles(PHEVs).

FIGS. 24A and 24B each illustrate an example of a vehicle using oneembodiment of the present invention. An automobile 8400 illustrated inFIG. 24A is an electric vehicle that runs on the power of an electricmotor. Alternatively, the automobile 8400 is a hybrid electric vehiclecapable of driving appropriately using either an electric motor or anengine. One embodiment of the present invention can provide ahigh-mileage vehicle. The automobile 8400 includes the power storagedevice. The power storage device is used not only for driving anelectric motor 8406, but also for supplying electric power to alight-emitting device such as a headlight 8401 or a room light (notillustrated).

The power storage device can also supply electric power to a displaydevice of a speedometer, a tachometer, or the like included in theautomobile 8400. Furthermore, the power storage device can supplyelectric power to a semiconductor device included in the automobile8400, such as a navigation system.

FIG. 24B illustrates an automobile 8500 including the power storagedevice. The automobile 8500 can be charged when the power storage deviceis supplied with electric power through external charging equipment by aplug-in system, a contactless power feeding system, or the like. In FIG.24B, a power storage device 8024 included in the automobile 8500 ischarged with the use of a ground-based charging apparatus 8021 through acable 8022. In charging, a given method such as CHAdeMO (registeredtrademark) or Combined Charging System may be employed as a chargingmethod, the standard of a connector, or the like as appropriate. Theground-based charging apparatus 8021 may be a charging station providedin a commerce facility or a power source in a house. For example, withthe use of a plug-in technique, the power storage device 8024 includedin the automobile 8500 can be charged by being supplied with electricpower from outside. The charging can be performed by converting ACelectric power into DC electric power through a converter such as anAC-DC converter.

Furthermore, although not illustrated, the vehicle may include a powerreceiving device so that it can be charged by being supplied withelectric power from an above-ground power transmitting device in acontactless manner. In the case of the contactless power feeding system,by fitting a power transmitting device in a road or an exterior wall,charging can be performed not only when the electric vehicle is stoppedbut also when driven. In addition, the contactless power feeding systemmay be utilized to perform transmission and reception of electric powerbetween vehicles. Furthermore, a solar cell may be provided in theexterior of the automobile to charge the power storage device when theautomobile stops or moves. To supply electric power in such acontactless manner, an electromagnetic induction method or a magneticresonance method can be used.

According to one embodiment of the present invention, the power storagedevice can have improved cycle characteristics and reliability.Furthermore, according to one embodiment of the present invention, thepower storage device itself can be made more compact and lightweight asa result of improved characteristics of the power storage device. Thecompact and lightweight power storage device contributes to a reductionin the weight of a vehicle, and thus increases the driving distance.Furthermore, the power storage device included in the vehicle can beused as a power source for supplying electric power to products otherthan the vehicle. In such a case, the use of a commercial power sourcecan be avoided at peak time of electric power demand.

This embodiment can be combined with any of the other embodiments asappropriate.

EXAMPLE 1

In this example, a manufacturing method of the positive electrode activematerial of one embodiment of the present invention is described.

Samples A1, C1, and C2 which are the positive electrode active materialsof one embodiment of the present invention were manufactured based onthe flow chart shown in FIG. 2. Note that in the case where the sameconditions were used for manufacturing the three samples describedbelow, the description will be omitted.

As lithium compound, LiCl was weighed to be 6.359 g in Step S201 a. Asthe phosphorus compound, H₃PO₄ was weighed to be 3.41 ml in Step S201 b.The number of moles of lithium was set to be three times that ofphosphorus. As the solvent, pure water was weighed to be 50 ml in StepS201 d.

Then, LiCl and H₃PO₄ were put into pure water, so that the mixedsolution A was formed in Step S205. Step S205 was performed in an airatmosphere. Note that while being stirred with a stirring means or thelike, materials and the like were put into pure water during theformation of the mixed solution.

Then, as the solution Q, ammonia water with a concentration of 28 wt %was prepared in Step S205 b.

After that, the solution Q was dropped into the mixed solution A and pHmeasurement was performed in Step S207. The solution Q is dropped untilpH becomes a desired one, so that the mixed solution B was formed. Here,pH of each sample was adjusted so that the concentrations of a mixedsolution C, described later, were the values shown in Table 1. For pHmeasurement, a SevenGo Duo pH meter produced by Mettler-ToledoInternational Inc. was used.

Then, as the M(II) compound, FeCl₂.4H₂O was weighed to be 9.941 g inStep S208. The number of moles of iron was equal to that of phosphorus.As the solvent, water was weighed in Step S209 b.

After that, each of the several types of mixed solutions B withdifferent pH was mixed with the mixed solution B, FeCl₂.4H₂O, and purewater, so that the mixed solution C was formed in Step S209. Table 1shows pH of the mixed solution C in each of the samples.

TABLE 1 Temperature [° C.] pH A1 180° C. 4.28 C1 150° C. 3.92 C2 150° C.6.5

Next, in Step S211, the mixed solution C was put into an autoclaveincluding an inner glass cylinder and was shut in and mixed. As forSample A1, heating was performed at 180° C. for one hour, as for SampleC1, heating was performed at 150° C. for one hour, and as for Sample C2,heating was performed at 150° C. for one hour. During heating, thepressure inside the inner cylinder was approximately 0.4 MPa to 0.5 MPaat 150° C. and approximately 0.9 MPa to 1.0 MPa at 180° C. After theheat treatment was performed, the heated mixed solution C was left untilthe temperature fell and the synthetic material inside the innercylinder was filtered and the residue was washed with water. For theautoclave, a mini reactor MS200-C manufactured by OM labotech Corp. wasused.

Next, the washed object was dried in a reduced-pressure atmosphere at60° C. for two hours, so that Samples A1, C1, and C2 including a powderyLiFePO₄ were obtained. Sample A1 had a gray powder, Sample C1 had aslightly darker gray powder than Sample A1, and Sample C2 had a slightlygreenish gray powder.

EXAMPLE 2

Analysis results of Samples A1, C1, and C2 manufactured in Example 1 aredescribed in this example.

<SEM Observation>

Each sample was observed with the use of SEM. The observation results ofSamples A1, C1, and C2 are shown in FIGS. 25A to 25C, FIGS. 26A and 26B,and FIGS. 27A to 27C, respectively. FIGS. 25A, 26A, and 27A showobservation results at a magnification of 50,000 times and FIGS. 25C,26C, and 27C show observation results at a magnification of 1,000 times.FIG. 25B is an enlarged view of a region surrounded by dotted lines inFIG. 25A, FIG. 26B is an enlarged view of a region surrounded by dottedlines in FIG. 26A, and FIG. 27B is an enlarged view of a regionsurrounded by dotted lines in FIG. 27A. The two particles included inSample A1 were measured in FIG. 25B and the minor diameters were 58 nmand 33 nm. The two particles included in Sample C1 were measured in FIG.26B, and the minor diameters were 478 nm and 665 nm. The two particlesincluded in Sample C2 were measured in FIG. 27B, the minor diameterswere 291 nm and 57 nm.

Hereinafter, the major diameter and the minor diameter of the particleswere calculated more specifically.

FIG. 1A illustrates groups 202 a, 202 b, and 202 c of the particles inthe observation result of Sample A1 illustrated in FIG. 25B. Particlesincluded in the groups 202 a, 202 b, and 202 c are schematicallyillustrated in FIG. 1B.

FIG. 28 shows an example of major diameters and minor diameters in thecase where the particles 203 a, 203 b, and 203 c shown in FIG. 1B wereapproximated to a rectangular shape. In the particle 203 a, a majordiameter 206 a was 352 nm, a minor diameter 207 a was 108 nm, and themajor diameter was 3.3 times longer than the minor diameter. In theparticle 203 b, a major diameter 206 b was 490 nm, a minor diameter 207b was 97.9 nm, and the major diameter was 5.0 times longer than theminor diameter. In the particle 203 c, a major diameter 206 c was 280nm, a minor diameter 207 c was 125 nm, and the major diameter was 2.2times longer than the minor diameter.

It is found that an excellent positive electrode active material withhigh aspect ratio and a small minor diameter can be obtained in SampleA1. On the other hand, the minor diameter of Sample C1 was more than orequal to 400 nm which is slightly large. There were conditions where theminor diameter of Sample C2 was as small as less than or equal to 100nm; however, as will be described below, the specific surface area waslarge and the particles were probably in contact with each other.

<Particle Size Distribution Measurement with Laser Diffraction andScattering Method>

Next, each sample was measured using laser diffraction particle sizeanalyzer (SALD-2200 manufactured by Shimadzu Corporation). A laserdiffraction and scattering method was used as a method for calculatingthe particle diameter. The measurement area of the device was more thanor equal to 0.030 μm and less than or equal to 1000 μm. The particlesize distribution measurement results of Sample A1, Sample C1, andSample C2 are shown in FIG. 29A, FIG. 29B, and FIG. 29C, respectively.Furthermore, the results of D50 and D90 of these samples are shown inTable 2. Here, D50 shows a particle diameter when accumulation ofparticles accounts for 50% of a particle size distribution curve in ameasurement result of the particle size distribution. In other words,D50 is a median. Furthermore, D90 shows a particle diameter whenaccumulation of particles accounts for 90% of a particle sizedistribution curve in a measurement result of the particle sizedistribution. Note that in the case where the same sample name is shownmore than once in Table 2, measurement was performed more than onceusing different powders.

TABLE 2 D50 [μm] D90 [μm] A1 2.196 34.073 A1 1.500 25.422 C1 4.63348.711 C1 4.437 43.238 C1 4.465 44.428 C2 13.806 51.055 C2 12.216 47.326C2 3.279 40.455

D50 in Sample C2 was more than 10 μm in some cases and such large valueswere larger than Samples A1 and C1 in Sample C2.

<Specific Surface Area Measurement>

Next, measurement of the specific surface area of each sample wasperformed. For the measurement of the specific surface area, amicromeritics automatic surface area and porosimetry analyzer (TristarII3020 manufactured by Shimadzu Corporation) was used. BET was used forthe analysis. Measurement results of the specific surface areas of thesamples are shown in Table 3.

TABLE 3 specific surface area [m²/g] A1 20.6777 C1 13.7049 C2 8.2877

Here, the density d=3.55 g/cm³ of LiFePO₄ was substituted to Formula (1)with use of the specific surface area of Table 3 to obtain the diameterin the case where Samples A1, C1, and C2 are approximated to a sphereshape; the calculated diameter of Sample A1 was 120 nm, that of SampleC1 was 181 nm, and that of Sample C2 was 299 nm.

As for Sample A1 formed with a formation temperature set to 180° C. andpH of the mixed solution C set to 4.28, the particle diameter obtainedfrom the specific surface area was roughly equivalent to the particlesize observed with SEM, which indicates that an excellent positiveelectrode active material with a small number of particles in contactwith each other was obtained. On the other hand, despite the presence ofa particle having a minor diameter less than or equal to 100 nm inobservation with SEM in Sample C2 formed with a formation temperatureset to 150° C. and pH of the mixed solution C set to 6.5 having a largerspecific surface area when compared to other conditions, it is suggestedthat a larger number of particles are in contact with each other.Furthermore, it is suggested from the analysis results of the particlesize distribution that the diameter of the group is large. Furthermore,the specific surface area of Sample C1 formed with a formationtemperature set to 150° C. and pH of the mixed solution C set to 3.92was larger than that of Sample C2. Hence, the adhesion between theparticles is expected to be prevented by shifting pH to an acidic side(a further small pH in acid).

This application is based on Japanese Patent Application Serial No.2016-135709 filed with Japan Patent Office on Jul. 8, 2016, the entirecontents of which are hereby incorporated by reference.

What is claimed is:
 1. A positive electrode active material comprising:a first group of particles; and a second group of particles, whereineach of the first group and the second group has a particle diameter ofmore than or equal to 300 nm and less than or equal to 3 μm, wherein thefirst group comprises a first particle and a second particle, whereinthe first particle and the second particle are each a lithium-containingcomplex phosphate comprising at least one of iron, nickel, manganese,and cobalt, wherein the first particle and the second particle each havea major diameter and a minor diameter, wherein the major diameters ofthe first particle and the second particle are substantially parallel toeach other, and wherein the major diameter of the first particle is twoto six times larger than the minor diameter of the first particle andthe minor diameter of the first particle is more than or equal to 20 nmand less than or equal to 130 nm.
 2. The positive electrode activematerial according to claim 1, wherein the positive electrode activematerial has an olivine structure.
 3. The positive electrode activematerial according to claim 2, wherein the positive electrode activematerial is represented by LiFePO₄.
 4. A power storage devicecomprising: a positive electrode comprising the positive electrodeactive material according to claim 1, and a negative electrode.
 5. Anelectronic device comprising the power storage device according to claim4.
 6. A positive electrode active material comprising: a plurality ofparticles comprising: a first particle; and a second particle, whereineach of the first particle and the second particle is alithium-containing complex phosphate comprising at least one of iron,nickel, manganese, and cobalt, wherein the first particle and the secondparticle each have a major diameter and a minor diameter in an uppersurface observed with a microscope, wherein the major diameters of thefirst particle and the second particle are substantially parallel toeach other, wherein the major diameter of the first particle is two tosix times larger than the minor diameter of the first particle and theminor diameter of the first particle is more than or equal to 20 nm andless than or equal to 130 nm, and wherein a median value of particlediameters of the plurality of particles obtained with use of a laserdiffraction and scattering method is more than or equal to 500 nm andless than or equal to 6 μm.
 7. The positive electrode active materialaccording to claim 6, wherein the positive electrode active material hasan olivine structure.
 8. The positive electrode active materialaccording to claim 7, wherein the positive electrode active material isrepresented by LiFePO₄.
 9. A power storage device comprising: a positiveelectrode comprising the positive electrode active material according toclaim 6, and a negative electrode.
 10. An electronic device comprisingthe power storage device according to claim
 9. 11. A positive electrodeactive material comprising: a plurality of particles comprising: a firstparticle; and a second particle, wherein each of the first particle andthe second particle is a lithium-containing complex phosphate comprisingat least one of iron, nickel, manganese, and cobalt, wherein the firstparticle and the second particle each have a major diameter and a minordiameter in an upper surface observed with a microscope, wherein themajor diameters of the first particle and the second particle aresubstantially parallel to each other, wherein the major diameter of thefirst particle is two to six times larger than the minor diameter of thefirst particle and the minor diameter of the first particle is more thanor equal to 20 nm and less than or equal to 130 nm, wherein a medianvalue of particle diameters of the plurality of particles obtained withuse of a laser diffraction and scattering method is more than or equalto 500 nm and less than or equal to 6 μm, and wherein a specific surfacearea is more than or equal to 18 m²/g and less than or equal to 50 m²/g.12. The positive electrode active material according to claim 11,wherein the positive electrode active material has an olivine structure.13. The positive electrode active material according to claim 12,wherein the positive electrode active material is represented byLiFePO₄.
 14. A power storage device comprising: a positive electrodecomprising the positive electrode active material according to claim 11,and a negative electrode.
 15. An electronic device comprising the powerstorage device according to claim
 14. 16. A manufacturing method of apositive electrode active material comprising: a step of mixing alithium compound, a phosphorus compound, and water to form a first mixedsolution; a step of adjusting pH by adding a first aqueous solution tothe first mixed solution to form a second mixed solution; a step ofmixing an iron(II) compound with the second mixed solution to form athird mixed solution; and a step of heating the third mixed solutionunder a pressure higher than or equal to 0.1 MPa and lower than or equalto 2 MPa at a highest temperature higher than 150° C. and lower than orequal to 250° C. to form a fourth mixed solution, wherein the positiveelectrode active material comprises a plurality of particles comprisinga first particle and a second particle, wherein pH of the third mixedsolution is more than or equal to 3.5 and less than or equal to 5.0,wherein each of the first particle and the second particle is alithium-containing complex phosphate comprising one or more of iron,nickel, manganese, and cobalt, wherein each of the first particle andthe second particle comprises a major diameter and a minor diameter inan upper surface observed with a microscope, wherein the major diametersof the first particle and the second particle are substantially parallelto each other, wherein the major diameter of the first particle is twoto six times larger than the minor diameter of the first particle andthe minor diameter of the first particle is more than or equal to 20 nmand less than or equal to 130 nm, and wherein a median value of particlediameters of the plurality of particles obtained with use of laserdiffraction and scattering method is more than or equal to 500 nm andless than or equal to 6 μm.
 17. The manufacturing method of a positiveelectrode active material according to claim 16; wherein the positiveelectrode active material has an olivine structure.
 18. Themanufacturing method of a positive electrode active material accordingto claim 17; wherein the positive electrode active material isrepresented by LiFePO₄.
 19. A manufacturing method of a positiveelectrode active material comprising: a step of mixing a lithiumcompound, a phosphorus compound, and water to form a first mixedsolution; a step of adjusting pH by adding a first aqueous solution tothe first mixed solution to form a second mixed solution; a step ofmixing an iron(II) compound with the second mixed solution to form athird mixed solution; and a step of heating the third mixed solutionunder a pressure higher than or equal to 0.1 MPa and lower than or equalto 2 MPa at a highest temperature higher than 150° C. and lower than orequal to 250° C. to form a fourth mixed solution, wherein the positiveelectrode active material comprises a plurality of particles comprisinga first particle and a second particle, wherein pH of the third mixedsolution is more than or equal to 3.5 and less than or equal to 5.0,wherein each of the first particle and the second particle is alithium-containing complex phosphate comprising one or more of iron,nickel, manganese, and cobalt, wherein each of the first particle andthe second particle comprises a major diameter and a minor diameter inan upper surface observed with a microscope, wherein the major diametersof the first particle and the second particle are substantially parallelto each other, wherein the major diameter of the first particle is twoto six times larger than the minor diameter of the first particle andthe minor diameter of the first particle is more than or equal to 20 nmand less than or equal to 130 nm, and wherein a median value of particlediameters of the plurality of particles obtained with use of laserdiffraction and scattering method is more than or equal to 500 nm andless than or equal to 6 μm, and wherein a specific surface area is morethan or equal to 18 m²/g and less than or equal to 50 m²/g.
 20. Themanufacturing method of a positive electrode active material accordingto claim 19; wherein the positive electrode active material has anolivine structure.
 21. The manufacturing method of a positive electrodeactive material according to claim 20; wherein the positive electrodeactive material is represented by LiFePO₄.